Evaluation of Mechanical, Wear and Corrosion Properties of Microcrystalline Coating on Titanium Alloy OT4-1

doi:10.21203/rs.3.rs-3103715/v1

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Evaluation of Mechanical, Wear and Corrosion Properties of Microcrystalline Coating on Titanium Alloy OT4-1

https://doi.org/10.21203/rs.3.rs-3103715/v1

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Thin-film barrier coating on titanium alloy substrate has a vast range of applications among which aeronautic and marine applications are significant. Talking about 2 very different aspects, whose requirements and applications are way apart. Titanium has always been beneficial in both these products’ applications. The introduction of titanium in aerospace and marine locomotive parts will have significant importance on parameters like pressure withstanding capacity and corrosion resistance. Four different specimens of OT4-1 Titanium alloy will be coated with Titanium Nitrate (TiN), Aluminium Chromium Nitride (AlCrNa pro), Diamond-like Carbon (DLC), and Tungsten Carbide Carbon (WCC). All the above mentioned are coated for 4 µm over the substrate. TiN and AlCrNa pro coatings are carried out in the Arc evaporation method, DLC coating was conducted using Plasma-assisted Chemical Vapour deposition and WCC coating was performed using sputtering process. Here an attempt has been made to improve Hardness, Wear and Corrosion by introducing a thin film coating on the base metal OT4-1.

Titanium Alloy

Microhardness

Pin-on-disc Wear

Electro-chemical Corrosion

Physical Vapor Deposition

Sputtering

Titanium amalgams, because of their fantastic blend of low thickness, high solidarity to weight proportion, great erosion obstruction and biocompatibility, are generally utilized in the aviation, auto, compound and biomedical ventures [1–3]. Titanium has a density of roughly 4.5 g/cm3, which is higher than that of other light metals of structural importance such as aluminium or magnesium but nearly half that of steels. Titanium also possesses good mechanical resistance to yield and fracture, an elastic modulus of roughly 115 GPa, and excellent corrosion resistance. Figure 1 shows the allotropic transformation in commercially pure titanium.

Titanium also has exceptionally low thermal conductivity and thermal expansion coefficients, about 22 W/mK and 84 * 10^− 6/c, which are nearly one order of magnitude and 1/3 of those of aluminium, respectively. Table 1 shows the properties of commercially available pure titanium.

Table 1

Properties of Commercially available Pure Titanium (theoretical)
Molecular Weight	47.86
Appearance	Slivery
Melting Point	1668^o c
Boiling Point	3560^o c
Density	4.54 g/cm³
Crystal Phase/ Structure	Hexagonal
Heat of Fusion	14.45 kJ/mol
Poisson’s Ratio	0.32
Specific Heat	0.125 Cal/g/K @ 25^o c
Tensile Strength	140 MPa
Thermal Conductivity	21.9 W/(m-K) @ 298.2 K
Youngs Modulus	116 GPa

1.1. Introduction to Titanium Wrought alloy OT4-1

At room temperature, the OT4-1 composition lies in two phases of the region in the Ti-Al-Mn phase diagram, and there is a strong partitioning of Al to the phase and Mn to the phase in the OT4-1 alloy, which stabilizes a small amount of phase in the microstructure, resulting in improved workability and strength. The evaporation of volatile substances from molten metal causes pressure buildup at the melt surface.

The strength of the OT4-1 alloys is nearly constant up to 2% and increases as the proportion of aluminum increases. Tensile characteristics at various annealing temperatures reveal a small loss in strength when annealed at 610°C and 690°C. The strength of the alloy which is controlled by grain size shows a linear relation with yield strength.

OT4-1 is a titanium composite utilized for high-temperature applications in the aeronautic trade. These materials are widely utilized in process enterprises, paper, and material plants, extractive metallurgy, the oil business, clinical sciences, and designing parts like steam and gas turbine sharp edges, associating bars driving rods, and so on. Thus, assessing its mechanical properties after coating the surface with different materials is very fundamental. In this way, concentrating on formability aspects is valuable further. The rising utilization of titanium composites in load-bearing components causes a requirement for examination into their solidarity qualities. [44]

OT4-1 is a low alloyed titanium combination handling a decent flexibility and low strength. The combination depends on the Ti-Al-Mn framework [39] and is expected for the most part for sheet semi-items, for example, primary and furthermore forgings, stampings, bars, shapes, lines, wire, and other semi-items. [35–38]

The chemical composition of the alloy OT 4 − 1 (Ti-Al-Mn system) is as follows (in weight percent): Al, 1.0–2.5; Mn, 0.7–2.5. The impurity concentration shall not exceed (weight percent) C, 0.10. [39] Table 2 shows the composition of the sheet.

Table 2

Chemical Composition of OT4-1
Fe	C	Si	Mn	N	Ti	Al	Zr	O	H	Impurity
max 0.3	max 0.1	max 0.12	0.7–2	max 0.05	94.33–97.5	1.5–2.5	max 0.3	max 0.15	max 0.012	other 0.3

A low alloy titanium alloy with high ductility and low strength is OT 4 − 1. The alloy, which is based on the Ti-Al-Mn system [7], is primarily designed for sheet semi-products, including structural goods, as well as forgings, stampings, rods, forms, pipes, wire, and other semi-products. [3–6]

The 1mm OT 4 − 1 sheet was received in the cool rolled and strengthened condition. The piece of the plates was affirmed by the Inductive coupled plasma optical spectrometer strategy and viewed as Russian Grade Titanium amalgam OT4-1. The elements of the Inductive coupled plasma optical spectrometer were:

Multi-element, a sequential technique with wavelength range: 190–750 NM
The Plasma produced by the electrode radio frequency discharge.
A High-resolution grating with RF source.
It can produce a very high temperature up to 10,000^o K.
The Capability of analysis from ppm to percentage level.

By using the deformation process, alloy OT4-1 is used to manufacture semi-finished products (sheets, strips, foils, plates, bars, rod, profiles, tubes, forgings and forged blanks), as well as ingots. Longitudinally welded pipes for industrial pipelines, operating at conventional pressure PN (Pu) not exceeding 10 MPa and temperature not exceeding 400^oc. The alloy OT4-1 is used for the manufacture of aircraft parts such as wing panels, flaps, and inner wing kits when it is in the annealed condition.

It is frequently necessary to alter the surface characteristics of titanium alloy products, for example, to enhance the alloy's solderability or change the product's look. The PVD process of depositing metals and alloys is one way to improve surface characteristics. The process of creating the product is greatly complicated by the hard oxide coating that covers the surface of titanium and its alloys.

2.1. PROCESSING TECHNIQUES

Physical Vapour Deposition is the general methodology used in coating the substrates. It is a process that takes place under high vacuum and, in most situations, at temperatures ranging from 150 to 500°C. In the PVD process, the high purity solid coating material (metals such as titanium, chromium, and aluminum) is either heated to a high temperature or blasted with ions (sputtering). A reactive gas is also injected at this time, such as a gas containing carbon or nitrogen, and it reacts with the metal vapour to generate a coating that is thin and very sticky and is then deposited on the substrate. By rotating the components along numerous axes at a steady speed, a homogeneous coating thickness may be achieved.

It is possible to carefully adjust coating qualities, including hardness, structure, chemical and temperature resistance, and adhesion.

Arc evaporation, Sputtering, Ion plating, and Enhanced Sputtering are examples of PVD techniques.

2.1.1. Arc Evaporation

In this procedure, the solid, metallic coating material is swept over by an arc with a diameter ranging from microns to only a few tenths of a micron, which causes it to evaporate. The nearly full ionization of the evaporated material as a result of the high current and power densities utilized results in the creation of a high-energy plasma.

When metal ions combine with a reactive gas injected into the chamber, they deposit as a thin, extremely adherent coating on the target tools or components.

Titanium Nitride (TiN) and Aluminum Chromium Nitride (AlCroNa Pro) coating was carried out using Arc Evaporation method. TiN coating specification are listed in the Table 3 and AlCroNa Pro specification are mentioned in the Table 4.

Table 3

Arc Evaporation specification for Titanium Nitride coating
Coating Material	TiN
Coating color	gold-yellow
Coating Hardness H_IT [GPa]	30 +/- 3
Coefficient of friction (dry) vs. steel	~ 0.6
Intrinsic stress [GPa]	-2 +/- 1
Max. service temp [^oC]	600
Process Temperature [^oC]	< 500

Table 4

Arc Evaporation specification of AlCroNa pro coating
Coating Material	AlCrN-based
Coating color	Bright grey
Coating Hardness H_IT [GPa]	36 +/- 3
Coefficient of friction (dry) vs. steel	~ 0.6
Intrinsic stress [GPa]	-3 +/- 1
Max. service temp [^oC]	1100
Process Temperature [^oC]	< 500

2.1.2. Plasma Assisted Chemical Vapour Deposition (PACVD)

This process produces metal-free carbon coatings using high frequency electric voltage. This procedure's setup is comparable to that of sputtering, which is usually combined with CVD. In PACVD, the vacuum chamber is filled with a gas containing the coating components, and an AC voltage-powered discharge is then ignited.

As a result, the tools and components are coated in a thick layer of free carbon and hydrogen atoms (ions and radicals). The applied voltage can be changed to affect the coating's qualities. Diamond like Carbon coating was carried out through PACVD method. Table 5 mentions the specification of the DLC coating.

Table 5

PACVD coating specification of DLC coating
Coating Material	a-C:H
Coating Colour	Black
Coating Hardness HIT [GPa]	~ 15–25
Coefficient of friction (dry) vs. steel	0.1–0.2
Max service temp [^oC]	300
Process temperature [^oC]	< 250

2.1.3. Sputtering

The material that will be coated in the sputtering process is heated first and then ion etched by being attacked with argon ions in the vacuum chamber. This results in a pure and clean metal surface devoid of atomic contamination, which is required for the optimal coating adhesion.

The coating material-containing sputtering sources are then subjected to a strong negative voltage. Positive argon ions are created as a result of the electrical gas discharge and are driven toward the coated material, where they atomize it. The hard coating's non-metallic component is subsequently reacted with vaporized, atomized metal in a gaseous phase.

In the end, a thin, compact covering with the correct structure and composition is deposited. Tungsten Carbide Carbon coating was carried out using Sputtering method. Table 6 depicts mentions the specification of WCC coating.

Table 6

Sputtering coating specification for WCC
Coating Material	a-C:H:Me (WC/C)
Coating Colour	Anthracite
Coating hardness H_IT [GPa]	10–15
Coefficient of friction (dry) vs. steel	0.1–0.2
Max. service temp [^oC]	300
Process Temperature [^oC]	< 250

2.2. Testing Methods

Microhardness is the mechanical test that was conducted to find out the hardness of the materials before and after coating. Corrosion testing assesses a material's corrosion resistance under a variety of environmental variables such as temperature, humidity, and salt water. The procedure used to assess the corrosion resistance of the substrates before and after coating was Electrochemical Corrosion resistance test. The outcome of a wear test is acquired from deformation, scratches, and indentations on the interacting surfaces. The wear test measures the changes in conditions caused by friction. The wear testing approach utilized to assess the wear rate of the titanium alloy before and after coating is pin-on-disc.

2.2.1. Microhardness

When a consistent compressive force is applied, a solid material's hardness may be calculated as a measure of its resistance to permanently changing form. Different methods, such as indentation, scraping, cutting, mechanical wear, or bending, might cause the distortion. Since strength, ductility, and fatigue resistance are all mechanical qualities that are closely related to hardness, hardness testing is a straightforward, quick, and reasonably priced way to check the quality of a material.

Vickers Hardness method is used to calculate the hardness of the coated and uncoated specimens. A pyramid-shaped diamond indenter creates an imprint under load that is measured as part of the Vickers hardness test. The indenter is a square-based pyramid with faces at 68^o, edges at 148^o, and opposing sides that meet at an angle of 136^o at the top.

The Vickers diamond hardness number, HV, is calculated using the actual surface area of the imprint Ac and the indenter load L.

HV = L/A_c = (2L/d²) sin(136^o/2) = 1.8544 (L/d²),

where L is measured in kgf and d (mm) is the diagonal length measured from corner to corner on the remaining impression in the specimen surface. To measure the size of the impression, which is normally no greater than 0.5 mm, a calibrated microscope with a tolerance of 1/1000 mm is employed. Vickers hardness is indicated as HV, and the units are commonly stated as kgf/mm² or MPa (the value in kgf/mm² multiplied by 9.8065).

2.2.2. Wear Test

Wear is a process of surface contact that results in material loss and surface deformation as a result of mechanical action between the sliding sides. Wear can also refer to the plastic deformation's dimension loss. The results of a wear test, on the other hand, are derived from deformation, scratches, and indentations on the interacting surfaces, and they quantify the changes in circumstances brought on by friction.

Pin on disk machine (DUCOM – TR 20-LE) as shown in the Fig. 2 was used to forecast the wear and friction behavior of the coated and uncoated Titanium Alloy samples OT4-1. The most common wear test procedures, according to research by Glaeser and Ruff, were pin-on-disc and pin-on-flat. Additional applications include material wear and friction of pin-on-disc, characteristics at high temperatures and under regulated the atmospheres. This testing procedure is pretty typical to apply thin coats. As per ASTM G99 guidelines, the tests were carried out at room temperature in an unlubricated setting. The parameters of the exam comprised, Load - Force values in Newtons at the worn contact; Speed - The relative sliding speed between the contacting surfaces expressed in meters per second or revolutions per minute. Time - The amount of time that the load is delivered on the specimen while the disc rotates at the sliding speed.

2.2.3. Corrosion Test

Corrosion is an unwelcome phenomenon that results in the destruction or deterioration of materials, having detrimental effects such as the closing of a plant, pollution from pipeline breaks, negative effects on the environment and human health, loss of productivity and products, tainted products from metal leaching, etc.

For a successful coating, proper coating application, including substrate preparation, is essential, and corrosion testing offers a quick way to assess how well your process is working. In conditions with high relative humidity, water can permeate coatings, causing blistering below as the substrate corrodes. Additionally, coatings can sustain degradation that causes filiform corrosion, the coating to separate totally from the substrate, or a combination of these.

2.2.3.1. Potentiodynamic Polarization Test

One of the most extensively used methods for gauging specimen corrosion is the potentio-dynamic polarization test. Between the working electrode and the reference electrode, the potential difference is measured. The two electrodes are subjected to a difference, and the current generated is measured. Pitting corrosion behavior is investigated using polarisation curves derived from the electrochemical system of a Gill AC unit used for corrosion testing. The basic electrochemical system with electrochemical fat cell employed in this investigation is depicted in Fig. 3. Programs to assess corrosion kinetic parameters like I_corr, E_corr, ba and bc, where ba and bc are the anodic and cathodic slope of the polarization curve, are included into this instrument itself.

3.1. Microhardness

Vickers microhardness method was used to evaluate the hardness values of OT4-1 before and after coating. The samples were subjected to 100 kgf force for 15 sec using the diamond indenter. The initial application of force is for 8 sec and then the test force is maintained for 15 sec. The hardness was sampled at 10 different locations across the metal for an average value of hardness. The measured hardness number is indicated as HV, which is Vickers hardness number. This can be converted to MPa by multiplying the number by 9.8065.

The base metal OT4-1 showed average hardness values of 334.35 HV or 3278.98 MPa without annealing at room temperature. Titanium nitrate coating on the base metal OT4-1 increased its hardness by 1.5 times with average hardness number of 500.72 HV or 4910.57 MPa. The hardness values of the base metal OT4-1 and OT4-1 after TiN coating is shown in the Fig. 4. X-axis shows 10 different readings across the sample and Y-axis shows hardness value in MPa.

AlCroNa Pro coating showed steady values of 875.7 HV or 8588.07 MPa with a maximum hardness value of 9288.40 MPa. The peak value is due to the presence of Aluminum with covers 29.5% weight of the coating. Figure 5 shows the graph comparing the hardness values of OT4-1 and AlCroNa Pro coated OT4-1.

Carbon based coating i.e., Diamond-like-Carbon and Tungsten carbide Carbon coating showed average values of 16140.28 MPa and 17086.02 MPa respectively. The hardness values have increased by 5.1 to 5.6 times the base metal OT4-1. But DLC coating showed irregular values ranging from 15367.56 MPa till 18540.13 MPa. WCC coating on the other hand shows stable values of microhardness. Figure 6 shows the microhardness values of DLC coating compared with OT4-1 and Fig. 7 depicts the microhardness values of WCC coating and OT4-1.

As it is clear Tungsten carbide carbo coating showed the highest hardness value compared to the other coatings. The reason behind this is based on how tungsten carbide is manufactured. Ball mills are used to combine powdered tungsten carbide and cobalt, and a binder ingredient is added to keep the powders together throughout the next stage of the process, which is compaction or pressing. During the compaction operations, hydraulic presses or isostatic presses are used to compact the powders into a shape that approximates the design of the finished product. Ordinary metalworking equipment may now be used to machine the powder compact. This is referred to as "Green Machining" at times. Fine powder particles must be removed with attention since they may pose a health concern; thus, effective extraction processes are required. After "Green Machining," the powder compact is ready to be sintered. Typically, this is done in a vacuum furnace at temperatures ranging from 1300 to 1600°C. During the sintering process, the tungsten carbide and cobalt matrix fuse together, resulting in a dense "Hard Metal". The material is so hard after sintering that it can only be machined by diamond grinding, a specialized kind of micro machining that is somewhat costly since it cannot remove huge amounts of material.

Furthermore, WC particles operate as internal balls that refine grain; therefore, hardness values are raised by increasing WC % owing to grain refinement, according to the Hall-Petch equation, which stipulates that particle size reduction improves hardness. Furthermore, the generated Cr-carbide phases improve toughness.

3.2. Wear rate

This section examines the wear characteristics of OT4-1 samples before and after coating when exposed to a 25 N load against a stainless-steel disc. The wear rate of the samples was determined using a pin on disc wear testing system supplied by DUCOM Instruments, Bangalore and purchased using funds from the Research Laboratory Upgradation Fund. The spinning hardened steel disc of 40 cm diameter, 0.11 µm surface roughness, and hardness of 840 HV is used in the pin on disc wear testing equipment (Fig. 2).

The samples were loaded on the horizontal arm of the apparatus under a load of 2.5 kg which is approximately 25 N. The machine was controlled to run the test for 10 minutes per sample with the disc rotating at a speed of 500 rpm. The results were in the form of graph with amount of Wear in micrometer at the X-axis and different data points in the Y-axis.

The equation W = M/ ρD is utilized in this research to compute the wear rate (mm³/m) of the samples based on the mass loss measurements acquired during the wear test. M is the mass loss in g, ρ is the sample density in g/cc, and D is the sliding distance in m. It was feasible to quantify the weight reduction of the pins by comparing the weight of the worn pins before and after the trials using an electronic scale with 0.01 mg accuracy. Before each experiment, the steel disc surface was carefully cleaned with acetone.

Table 7

Weights of the sample before and after the wear test.
Sample name	Weight before wear test (g)	Weight after wear test (g)	Difference in weight (g)
OT4-1	1.9476	1.848	0.0996
TiN coating OT4-1	2.6	2.555	0.045
AlCroNa Pro coated OT4-1	2.676	2.639	0.037
DLC coated OT4-1	2.56	2.522	0.038
WCC coated OT4-1	2.710	2.686	0.024

Volume of the sample need to be evaluated in 2 steps, below Fig. 8 shows the sketch of the sample used for the wear test. The volume should be calculated for the first rectangle on top then the volume should be calculated for the rectangle below. Volume can be calculated by the formula, V = l × b × h, where l is the length in cm, b is the breath in cm and h is the height in cm (0.1 cm).

Volume of the first rectangle will be -

V₁ = l × b × h

V₁ = 1 × 0.8 × 0.1

V₁ = 0.08 cm³

Volume of the remaining part of the sample will be –

V₂ = l × b × h

V₂ = 2 × 2 × 0.1

V₂ = 0.4 cm³

Total Volume of the sample will be the sum of the volumes of the individual parts –

Total Volume = V₁ + V₂

V = 0.08 + 0.4

V = 0.48 cm³

To calculate the density of the sample, we use the formula ρ = M/V, where M is the initial mass of the samples and V is the volume of the sample. Table 8 shows the densities of the different samples calculated using their initial weight.

Table 8

Densities of the different samples used in wear test
Sample	Density (g/cc)
OT4-1	4.0575
TiN coated OT4-1	5.416
AlCroNa Pro coated OT4-1	5.575
DLC coated OT4-1	5.3458
WCC coated OT4-1	5.6458

To calculate the sliding distance can be calculated using the formula D = πdN/1000, where d is the length of the specimen in contact with the disc in cm, N is the load applied on the pin in N. The load applied on the pin was standard throughout the experiment, 25 N. The length of the sample that was in contact with the disc in 2cm.

Therefore, D = πdN/1000

D = π × 2 × 25/ 1000

D = 0.15707 cm

Now, to calculate wear rate, we have all the variables, i.e., weight lost in g, Density in g/cc and Sliding distance. Applying the variables to the equation W = M/ρD, we get the wear rate of the specific samples in mm³/m.

Table 9

Wear rate observed in different specimens
Sample	Wear rate (mm³/m)
OT4-1	0.15635
TiN coated OT4-1	0.05292
AlCroNa Pro coated OT4-1	0.04227
DLC coated OT4-1	0.04527
WCC coated OT4-1	0.02707

By examining the wear rate of the samples obtained in Table 9, it appears that Archard's law was broken, which stipulates that the hardness of the material is one of the most significant elements of the wear rate [44]. The wear volume may be calculated using the Archard equation W = k × Wn × s/H, however one issue is the hardness H of the softer material at the sliding interface. Because wear at the boundary/mixed or hydrodynamic regime only occurs in the few microns range, this hardness is difficult to quantify, because bulk hardness is involved utilizing Vickers and Rockwell techniques, and the top surface generally experiences strain hardening while sliding.

Figure 10,11,12 and 13 show a comparison graph of wear in the coated and uncoated samples of OT4-1, respectively.

When the wear losses of samples are compared, it is discovered that the losses in coated specimens are fewer than those in uncoated specimens. DLC coating has the lowest surface roughness of the coatings. The major source of this outcome is the change in gas fluxes during the coating process. When the acquired results are compared, the order of the samples' average surface roughness values is TiN > DLC > AlCroNa Pro > WCC. The wear widths on the samples were found to be uniform and almost parallel to each other. As a result, the wear rate of the wear track is used to calculate the amount of wear in the specimens.

The friction force in the WCC and AlCroNa pro coating is 5.3 N which is almost 1.5 times lesser than the uncoated sample, 6.8 N. This result aligns with the results we obtained above.

3.3. Corrosion Resistance

Using the electrochemical workstation, Gill AC with ACM instrument version 5 software, the electrochemical measurements were performed in a non-stirred environment without aerating the solutions. The moulded test coupon was used as the working electrode, the platinum electrode served as an auxiliary electrode, and the saturated calomel electrode served as the reference electrode in a typical three electrode cell. Test coupon was exposed to potentio-dynamic polarisation tests without any surface treatment.

The medium in which the test conducted was 3.5% NaCl solution. The usual salt content of sea water is 3.5%. Since NaCl is the most prevalent and prolific corrosive chemical on earth. Additionally, the 0.6 molar NaCl solution is recognized by ASTM standard. The weight percentage of a 0.6 molar solution is again 3.5%.

The specimen was polarised to -300 mV catholically and + 1000 mV anodically with respect to the OCP at a scan rate of 21.6727 mV/min to capture the potentio-dynamic current-potential curves. The specimens were induced in the test for a time period of 60 minutes.

Evident shifts in the anodic and cathodic branches occur at lower corrosion current density (I_corr). Additionally, in comparison to the blank, the corrosion potential (E_corr) changes to the cathodic side. To guarantee accuracy, the linear Tafel areas of the cathodic curves were extrapolated to OCP in order to calculate the corrosion current densities (I_corr), as the anodic branches in NaCl electrolytes do not show linear Tafel regions [46].

Electrochemical polarization parameters, like E_corr, I_corr, I_corr(inh), cathodic Tafel slope (-β_c) calculated from the Potentio-dynamic polarization studies in given medium are presented in Table 10.

Table 10

Polarization data for the corrosion of OT4-1 samples before and after coating in 3.5% NaCl solution
Sample	E_corr (mV)	I_corr (µA/cm²)	I_corr(inh) (µA/cm²)	-β_c (mV/dec)
OT4-1	299.27	0.5589	0.1542	189
TiN coated OT4-1	277.62	0.6435	0.0655	139
AlCroNa Pro coated OT4-1	220.13	1.2686	0.04963	134
DLC coated OT4-1	271.15	3.9875	0.113	137
WCC coated OT4-1	255.23	2.375	0.06021	134

The rate of corrosion V_(corr) was calculated using this equation -

V _(corr) (mm/year) = K × I_corr × M/ ρ × n ….. (1)

where K denotes corrosion rate and is equal to 0.00327 mm g A^− 1 cm^− 1 y^− 1, establishing the unit of corrosion rate in millimetres per year. I_corr is the current density in A cm², is the density of the corroding specimen, M is the molar mass, and n is the metal's valence.

The inhibition efficiency (η) of NaCl solution was determined as the function of surface coverage (θ) using this equation –

η (%) = θ × 100 ….. (2)

The surface coverage (θ) is computed using –

θ = I_corr - I_corr(inh) / I_corr ….. (3)

To calculate Molar Mass of the sample, Moles = Mass / RAM (assuming percentage composition). OT4-1 constitute mainly of Titanium (Ti), Aluminum (Al) and Manganese (Mn). Molar mass of Ti = 47.9 g/mol; Al = 26.98 g/mol; Mn = 54. 94 g/mol.

Percentage of the above-mentioned elements in the alloy are – Ti = 95.5%, Al = 2.5%, Mn = 2%. Therefore, the masses of the elements are – 95.5 g is Titanium, 2.5 g of Aluminum and 3 g of Manganese.

By applying the molar mass formula to the above elements, we get –

MM (Ti) = Mass/RAM

MM (Ti) = 95.5/47.9

MM (Ti) = 1.997 mol

Similarly, MM (Al) = 0.0926 mol and MM (Mn) = 0.0364 mol. Hence the total Molar Mass of the substrate is –

MM = MM (Ti) + MM (Al) + MM (Mn)

MM = 1.997 + 0.0926 + 0.0364

MM = 2.126 mol

The valency of an element is When an element creates chemical compounds or molecules, its ability to combine with other atoms is measured by its valence or valency. Valency of Titanium is 4, Valency of Aluminum is 3 and Valency of Manganese is 7. Hence the total valency of the alloy is –

Valency (n) = V (Ti) + V (Al) + V(Mn)

Valency (n) = 4 + 3 + 7

Valency (n) = 14

Now using Eq. (1), Corrosion rate can be calculated by substituting all the variables.

Equation (3) depicts the surface coverage (θ) formula, which can be calculated using values in Table 10.

θ = I_corr - I_corr(inh) / I_corr

θ_(OT4−1) = 0.5589–0.1542 / 0.5589

θ_(OT4−1) = 0.4047/0.5589

θ_(OT4−1) = 0.7241

Similarly, θ_(TiN) = 0.8989, θ_{(AlCroNa Pro)} = 0.9608, θ_(DLC) = 0.9716, θ_(WCC) = 0.9746. Table 11 shows the corrosion rates and inhibition efficiency of the coated and uncoated samples.

Table 11

Corrosion rate and Inhibition efficiency of the coated and uncoated substrates in 3.5% NaCl solution
Sample	Corrosion Rate, V_(corr) (mm/year)	Inhibition efficiency, η (%)
OT4-1	5.84 × 10^− 5	72.41
TiN coated OT4-1	6.90 × 10^− 4	89.89
AlCroNa Pro coated OT4-1	1.115 × 10^− 4	96.08
DLC coated OT4-1	3.703 × 10^− 4	97.16
WCC coated OT4-1	2.113 × 10^− 4	97.46

Figures 14,15,16 and 17 compares the Tofel plot of the uncoated sample of OT4-1 with each of the coatings on the base metal. The red and grey lines are the results of Uncoated OT4-1 metal and blue, purple and orange line are the results of coated OT4-1.

Figure 22 depicts the Tafel curves of the OT4-1 with an AlCroNa Pro coating and after an electrochemical corrosion test. The best corrosion resistance performance was indicated by the sample's minimum corrosion potential and maximum passive current density at room temperature. The polarisation curves revealed that Titanium Nitrate coating had the lowest corrosion resistance due to its lowest corrosion current density and highest corrosion potentials.

As per the earlier examination, the primary part of as-stored AlCroNa Pro coatings was a fcc-(Cr,Al)N stage, as confirmed by the coatings' plentiful beads and particles. After cooling to room temperature, the amount of particles that drops on the outer layer of the (Cr,Al)N stage radically decreased as it split into hcp-AlN and CrN stages. making actual touch with the substrate. hcp-AlN, - FeCr, and minor amounts of CrN, Cr₂N, and Al₂O₃ stages made up most of these coatings right now. The examples covered with AlCroNa Pro had the most elevated corrosion resistance since it is notable that the hcp-AlN, Al₂O₃, CrN, and Cr₂N stages major areas of strength for having obstruction [45].

This chapter is the compilation of different mechanical and chemical properties of the Titanium alloy OT4-1. The coating characteristics have also been studied by coating the samples with Titanium Nitrate, AlCroNa Pro, Diamond like Carbon, Tungsten Carbide Carbon. The respective coated samples have been tested out for various tests like Microhardness, Wear Resistance and Corrosion Resistance. The project also compares the results obtained from the coated sample and uncoated sample, comparing them and suggesting a suitable coating methodology and coating material for different use purposes. The concluded points of the results are as follows-

4.1. Microhardness

Microhardness of the samples were tested using Vickers Hardness method. The diamond indenter produced a force 100 kgf for 15 seconds on the sample, the diagonals lengths of the indentation produce the hardness value of the samples in HV. The uncoated sample showed an average value of 3278.98 MPa, which a bit lower than the industry standard titanium alloy Ti-6AL-4V which has a hardness value of approximately 3600 MPa. TiN and AlCroNa Pro coating showed gradual increment in hardness value at 4910.57 MPa and 8588 MPa respectively. These coating showed an improvement in hardness of almost 1.5–2.6 times the base metal OT4-1. WCC and DLC coating showed extreme hardness with an average value of 17140.28 MPa and 16068.2 MPa respectively. These coating increased the hardness of the base metal by at least 5 times. These levels of hardness are very difficult to process when coated on large equipment’s. the Fig. 18 shows the graph of microhardness value of all coating in comparison with the base metal OT4-1.

4.2. Wear Resistance

The coated and uncoated samples of Titanium alloy OT4-1 were tested for wear resistance using Pin-on-disc wear testing machine. The specimens were subjected to 2.5kg or approximately 25 N of force on a rotating disc of 40 cm diameter which was running at a speed of 500 rpm for 10 minutes. Following the results of microhardness Tungsten carbide carbon was the most wear resistant coating, with a wear rate of 0.02707 mm³/m, which is almost 6 times more wear resistant than the uncoated sample OT4-1, which showed a wear rate of 0.15635 mm³/m. AlCroNa Pro coating showed the next best wear rate of 0.04227 mm³/m/, which is approximately 3.7 times more wear resistant than uncoated sample OT4-1. This coating lost less weigh when measured after the testing compared to DLC coating. Therefore, the most wear resistant coating were Tungsten Carbide and AlCroNa pro followed by DLC and TiN. Figure 19 shows a graph of all coating on OT4-1 in order of its wear rate from lowest to highest.

4.3. Corrosion Resistance

Titanium and its alloys do not show any type of rust until at least 1200 hours of them being exposed to corrosive medium. Since replicating this is difficult, an Electrochemical workstation was used to test the corrosion resistance of the samples. The medium in which the test conducted was 3.5% NaCl solution. This specimen was polarised to -300 mV catholically and + 1000 mV anodically with respect to the OCP at a scan rate of 21.6727 mV/min. the potentio-dynamic polarisation curves obtained after testing showed that AlCroNa Pro followed by WCC coating consumed the most current. The same two coating showed least corrosion rate of 1.115 × 10^− 4 and 2.113 × 10^− 4 respectively in comparison with the uncoated sample OT4-1. The coating of AlCroNa pro improved the base metals’ corrosion resistance by 47.6%. Figure 20 shows the Tofel graphs of all coated specimens in comparison with uncoated sample OT4-1. Red lines are the uncoated OT4-1, pinks ones are TiN coated OT4-1, grey describe the DLC coating, yellow lines are WCC coating and blue is the AlCroNa Pro coating.

Ethical Approval

Not applicable

Competing Interests

Not applicable

Authors Contributions

Dr. K. Chandrappa provided us with the raw materials and helped in choosing the coating material for the same.

Anush and Anoop conducted studies on the raw material before and after coating. Anoop drafted the manuscript and prepared all the calculations required for the results. Anush added all the images and tables required in the research paper.

Funding

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Availability of data and materials

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Evaluation of Mechanical, Wear and Corrosion Properties of Microcrystalline Coating on Titanium Alloy OT4-1

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Abstract

Figures

1. INTRODUCTION

1.1. Introduction to Titanium Wrought alloy OT4-1

2. MATERIALS AND METHODOLOGY

2.1. PROCESSING TECHNIQUES

2.1.1. Arc Evaporation

2.1.2. Plasma Assisted Chemical Vapour Deposition (PACVD)

2.1.3. Sputtering

2.2. Testing Methods

2.2.1. Microhardness

2.2.2. Wear Test

2.2.3. Corrosion Test

2.2.3.1. Potentiodynamic Polarization Test

3. RESULTS AND DISCUSSION

3.1. Microhardness

3.2. Wear rate

3.3. Corrosion Resistance

4. SUMMARY AND CONCLUSIONS

4.1. Microhardness

4.2. Wear Resistance

4.3. Corrosion Resistance

Declarations

References

Additional Declarations

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