The poor wettability between graphite phase and copper phase can lead to the low bonding strength between them,which affects the properties of G/Cu(graphite/copper) composites.The addition of CCG not only enhances the bonding strength between graphite and copper phases,but also forms a more complete network structure between copper phases.
3.1 Mechanical properties
The mechanical properties of G/Cu composites have a certain influnce on the triblology behavior of composites.When the friction pair is fixed,mechanical properties affect friction mechanism of the composites,like the forming speed of friction film and the wear rate.Under normal circumstances,when the hardness of composite material is high,it will lead to serious abrasive wear at the beginning of the experiment.When the mechanical property is poor,it will have a deep impact on the fatigue wear at the later stage,resulting in a higher wear rate.The mechanical properties of the CCG-G/Cu(copper-coated graphite-graphite/copper) composites are listed in Fig.1,and the CCG ranges from 0wt% to 50wt%.The results show that because of the low strength and large particle size of CCG,the hardness and bending strength of the composites decrease with the increase of CCG content.
3.2 Micro-morphology of composites
The microstructure and morphologies of G/Cu composites and CCG-G/Cu composites are shown in Fig.2.Mainly studied the composites with CCG from 0wt% to 50wt%,and the total graphite phase content remains unchanged at 50wt%.Fig.2(a)(G/Cu composites) and Fig.2(b)(CCG-G/Cu composites) show that the copper and graphite are more easy to agglomerate because of the density difference,the splitting effect of the graphite on the copper phase prevents the copper phase from connecting to each other.There are fractures in the network structure formed by the connection between the copper phase in the graphite-rich area.As the content of CCG continues to increase,the copper coating and CCG which with large particle size contribute to the formation of more complete and continuous network copper structure.It formed the conduction and thermal channels,which can dissipate the friction heat generated in the friction process.
3.3 Tribology behavior
3.3.1 Friction coefficient and wear rate
After 300min ring-block friction and wear experiment,the friction coefficient and wear rate of the G/Cu composites and CCG-G/Cu composites are shown in Fig.3.The friction coefficient and wear rate of two kinds of materials show that the CCG content makes a big difference on tribological properties of the composites.As the content of CCG increases from 10wt% to 50wt%,the friction coefficient of the composites increases from 0.203 to 0.207 first and then decreases to 0.16,the friction coefficient of the composites is basically unchanged when the addition amount of CCG is 10wt%,and then decreases sharply when its content increase to 20wt%.The content of the CCG with large particle size on the surface is lower when the addition amount is small,it has little effect on the composition and integrity of the friction film on the surface.With the continuous increase of its content,the content of it spread on the surface increases,and it is more easily to fall of between layers and the coating under the action of the friction force during the friction process.
3.3.2 Micro-structure and micro-morphology of worn surface and debris
The information provided by the micro-morphology of worn surface and debris is helpful to infer wear mechanism during the friction process.The differences of friction micro-structure and composition cause to occur different friction mechanism,and eventually different surface micro-morphology is formed.SEM images EDS(Energy Disperse Spectroscopy) patterns show that compared with other kinds of composites(Fig.4(a)),20wt%CCG-G/Cu composites have friction film with fewer cracks and pits but abundant grooves along the frictional dicretion is formed on the worn surface.There are almost no abrasive chips with large particle size on the worn surface,and its total number is much less than that of other two kinds of composites.Its low hardness leads to hard microbumps scratch the sample under shear action and leave scratches on the surface,so the abrasive wear is more likely to occur on the friction surface in the initial friction process.This leads to abundant grooves produced on the friction surface caused by abrasive wear,but with few surface cracks and large holes caused by adhesive wear and fatigue wear on the surface.And the picture also shows that the friction film is well combined with the matrix,and it can effectively maintain the integrity and inhibit the renewal rate of the friction film.The SEM micrograms of 40wt%CCG-G/Cu composites(Fig.4(c) and (f)) show that the friction film formed on the contact surface is complete but has lower bonding strength with the matrix,there are a mass of long cracks on the surface,and there is a serious tendency to fall off.
To further verify the above conclusions,Tecnai G2 F20 type TEM instrument was used to characterize the crystal structure,micro-structure and micro-morphology of debris.
The microstructure and morphology of the debris of the G/Cu composites (Fig.5(a~c,f))and 20wt%CCG-G/Cu composites(Fig.5(d~e,g~i)) are shown in Fig.5.Morphology image of G/Cu composites (Fig.5(a)) show that the debris have various forms,some of which are formed by interlayer shedding of surface graphite under the action of shear force,and some of which are formed by rolling the accumulation fine chips under the action of furrow cutting,and some large particle size of the debris that fall off under the action of fatigue wear.The exfoliated lamellar graphite is relatively neat and its surface is flat without obvious folding.And its HRTEM image implies that its structure and the component is pure graphite,and due to the continuous cyclic stress,the graphite expanded and the spacing of the layers widened.The composition of the grinding chips near the red frame in Fig.5(a) is not uniform,and the edge of the grinding chips is relatively smooth and irregular,and the grinding chips are roughly elliptic.It can be inferred that the composition here is relatively complex,and graphite,copper and copper oxide coexist.Its HRTEM images(Fig.5(c and f)) display a great number of structural deformation areas and structural defects,and the direction and degree of deformation are different.Most of the deformation areas exist near the grain boundary,indicating that the debris is simply piled up by various components of finely grinding chips,and the deformation in this area is formed because the metal phase is plastic deformed under the action of friction force and friction heat,and then transferred to the contact surface,deformation degree in the edge of the debris is deeper because the spacing between the accumulated finely grinding chips is large,and it is easy to generate cracks here and extend along the friction direction,and finally fall off,and the results of Fast Fourier Transform(FFT) also show that there are many kinds of structural crystals in this region.Oxidative wear mainly occurs when there is exposed copper on the surface of the sample and friction pair,once a relatively complete friction film is formed on the surface,the possibility of oxidation wear is reduced.Adjacent copper and its oxide are easily connected to each other by plastic flow,part of the connecting metal oxides covered the graphite surface,which damaged the integrity of the graphite layer on the friction surface.Because of the poor wettability between copper phase and graphite phase,lamellar graphite structure near the interface cannot be stretched and tiled on the surface,which led to shrinkage,folding and even being torn.The accumulation of the damaged graphite,copper and its oxides caused the complex composition and structure of the area.
The TEM micrograms of the debris of the 20wt%CCG-G/Cu composites(Fig.5(d and e)) show that there are two kinds forms of debris:As shown in Fig.5(d),lamellar graphite with regular edges is formed by interlayer shedding of surface graphite particles under shear force.Fig.5(b) shows a debris accumulation layer with relatively smooth edges and uneven structural composition,and the content of such debris metal and its oxides is relatively high.The HRTEM image(Fig.5(g)) of the edge area of chip graphite in Fig.5(d) show that there are other crystals in this region besides graphite,graphite structure is destroyed under the effect of friction shear stress,causing graphite layer spacing increases,and leading to interlaminar fracture,finally formed the pieces of graphite,even form the disordered carbon material,as shown in І area.The HRTEM images of the accumulated debris in Fig.5(e) show that the content of copper and its oxide is high,the evidence of graphite is not obvious and there are a lot of deformation areas at the edge of the debris.The deformation areas with different sizes and deformation directions are attached to each other,and the grain boundaries with different spacing are formed.The degree of deformation determines the state of boundary connection,a wide crack between the two phases is formed when the crystal orientation of adjacent regions is different and the deformation degree is deep,and the two adjacent different phase are easy to be separated under the action of cyclic stress,cracks developed on the friction surface,which damage the surface friction layer.
The composition and crystal structure of the friction surface are similar to the debris,the SAED(Selected Area Electron Diffraction) spectra(Fig.6(a)) shows the crystal plane infromation of the diffracted crystal of friction,the presence of the copper oxides(such as CuO and CuCO3)clearly show that oxidative wear occurs during the experiment,the copper and graphite phases are oxidized by oxygen in the air to form oxygen-containing compounds such as CO2,CuO,Cu2O and CuCO3 when the surface temperature rises under the action of frictional heat.The high graphite content in the samples results in a complete and thick friction film on the contact surface.The morphological images of the G/Cu composites(Fig.6(b)) and the CCG-G/Cu composites(Fig.6(e)) show that the friction film made by stacking and pressing fine grinding chips of different sizes is relatively complete.The shape of the friction film formed by stacking is different,and the stacking direction is different,and the thickness is also significantly different.The friction film of the G/Cu composites is accumulated by fine grinding chips of different shapes and sizes layer by layer.The shape of the fine grinding chips in this area is relatively regular,and most of them are elliptic lamellae with large particle size range and smooth edges.Based on the above analysis,it is speculated that most of them are composed of copper and its oxide.Some sharp and neat long strip chips also can be seen on the surface,which are narrow and short,it may be formed by interlayer shedding of small particle size graphite or by cutting the surface carbon film for abrasive wear.The HRTEM images of the edge of the debris show that the graphite content is relatively high there,and the crystal structure of graphite is destroyed under the action of stress and shear force and some of them turns into disordered structure carbon.Because of poor bonding strength between graphite phase and copper oxides in debris,the cracks are more likely to occur there under the action of cyclic stress.The Fig.6(f~h) show that a large number of deformation areas with different degrees and directions exist in the copper phase enrichment area,and the enlarged view of the deformation area show that there are wide boundary gaps at the edge of the deformation area.As the experiment go on,the cracks are derived and extended under stress,causing local shedding of the friction layer.Due to the structural defects of the samples and the low strength of the friction layer,it is easy to go through the process of generation - enrichment - shedding - regeneration of the friction film in this area.
In contrast,the composites added 20wt%CCG has better friction and wear performance,the friction coefficient and wear rate of it is lower than G/Cu composites.This is because the friction film formed after the sheets of the graphite falling off from the surface of CCG are tiled on the friction surface has higher graphite content and better integrity,and the friction coefficient of composites is effectively reduced.Appropriate content of CCG(20wt%) can ensure that the friction surface has a certain strength,and the bonding strength with the matrix is not too weak.The thickness of the friction film formed is moderate,and the renewal speed of the friction layer is not too fast.Therefore, the wear rate of the composites is low.
3.3.3 Micro-structure and micro-morphology of friction cross section
The change of crystal structure,morphology and composition before and after the test can be compared more intuitively through the TEM images of friction cross section.And the friction layer,the friction secondary layer and the sample matrix can be clearly observed.
For G/Cu composites,its TEM micrograms of the friction cross section (Fig.7) show that there are deformation zone and non-deformation zone(matrix) in the longitudinal direction,and there is a great difference in the morphology and structure of the two regions.The structure of the matrix is uniform,and the graphite phase and copper phase maintain their complete structure without deformation.It can be seen from Fig.7(c) that the interface of the two phases is poor in wettability,resulting in low bonding strength of the two phases.The morphology image of the composites shows that the contact surface is seriously deformed after friction and wear test,and the graphite and copper phases are deformed and cross-linked under cyclic stress.Interlaminar shedding and intramolecular tearing occurs in graphite under shear action.Small particle size graphite from the orignal no specific direction the friction direction under the action of friction force.The change of the direction of each phase results in the increase of the spacing between particles and the cracks are easy to be derived,the softening plastic flow of copper phase flows into the cracks between the phases.Fig.7(e) shows that there are small pieces of metal and metal oxide phases between the graphite phases,and a mass of dislocation areas exist in the metal phase area.The partial enlargement in Fig.7(e) shows that this area is composed of a large number of crystal structures with different orientations,and there are obvious grain boundary between different crystals.The boundary spacing varies with the crystal structure and deformation degree,and there are obvious boundary regions between graphite phase and metal phase due to low binding strength,part of the graphite structure in the edge region is completely destroyed and becomes an invisible carbon structure.
In a word,the crystal structure of the friction surface changed after the friction and wear test.The graphite phase fell off between layers and tore within layers under the action of the frictional force,and the laminated deformed graphite lay flat on the friction surface to form a carbon film,which reduced the friction coefficient.Because of the softening of copper phase,a mass of deformation zones are formed due to plastic deformation.Due to different degrees of deformation,grain boundary cracks with different sizes are formed among different phases.When serious deformation occurs,wide cracks are formed and continue to extend until spall occurs under the action of cyclic stress.The main wear mechanism at this stage is fatigue wear.
For CCG-G/Cu composites,the TEM and HRTEM micrograms of its friction cross section show that because of the special contact mode of ring block friction and wear,the friction surface has different deformation strength under different stress time,the Fig.8(a) shows that the deformation area presents a triangular trend,and the deepest deformation can reach 2~3μm.The length of the deformation zone,crystal structure and composition on both sides of the center position are different due to the different force direction and magnitude.Along the direction of friction,the deformation is divided into the accumulation zone and the drag zone with the lowest point as the center.In front of the friction direction,attribute to the serious accumulation of grinding debris,the deformation zone is small,the degree of deformation is deep,and the crystal structure at the edge of the deformation area is more complex.In order to further study the difference of the structures in each region,the friction film Ӏ,stacking area Ⅱ and dragging area Ⅲ are analyzed with high resolution analysis method.
The HRTEM image(Fig.8(a)) of the friction film show that a heavily deformed thin layer of graphite is spread on the friction surface,and the graphite layer spacing expands under the cyclic stress.Close to the friction film area,the structure is damaged in a deep degree and independent massive metal oxides which formed by oxidation of copper appear.As the stress concentration point,the bonding effect between copper and other surrounding phases is destroyed and form a wide space,leading to the surface carbon film is prone to fall off.The graphite is extruded to the surface under shear action.The morphologies and structures of zone І(Fig.8(c)) and zone Ⅱ(Fig.8(d)) are similar,there are abundant metal rich areas with a lot of deformation areas,and the grain between graphite phase and metal phase are obvious.The graphite content is high in the boundary area,and the structure has been seriously damaged under the external force,the interlaminar strength of graphite decreased and the interlaminar shedding occurred.The split of the oxide of copper broke the graphite and formed graphite fragments,even broken into amorphous carbon structures.In comparison,the graphite content is higher in the accumulation zone,because the graphite density is low and the graphite is easy to be transferred over a long distance until it encounters an uphill slope ahead of the friction on the friction surface,leading to the accumulation of grinding chips in this area.
3.3.4 Analysis of friction mechanism
Understanding the friction mechanism of composite materials can effectively take corresponding measures to improve the friction and wear properties of composites materials,and reducing the wear amount of composites and increasing the service life of composites.Based on the above information,the friction mechanism of the composites in the friction process is analyzed as shown in Fig.9.
For the ring-block friction and wear mode,due to its special contact mode,the actual contact area between the friction pair and the sample surface is small at the initial stage,and there is only a thin film because of physical adsorption on the contact surface.Under the action of external force,the contact micro bump is easy to destroy the physical adsorption film and enter the contact surface,and formed furrows on the surface under the action of friction.Friction heat is easily generated in the friction process,which increases the temperature of the contact surface.When the temperature rises,the exposed copper phase is prone to be oxidized by oxygen in the air,thus forming an oxide film on the contact surface,the oxide film with complete morphology is helpful to reduce the friction coefficient of the composites.At the same time,the exposed copper phase is softened and adhesive wear occurs as the surface temperature rises,and point pits tend to be formed on the surface.As the experiment continued,the copper phase on the surface was covered by oxide film and fine debris,which reduced the probability of oxidation wear and adhesive wear.Meanwhile,the graphite phase deformed under the action of cyclic stress,and finally fell off to form lamellar graphite,which was transferred to the surface of the sample and friction pair to form a friction film with high graphite content,and the friction coefficient of the composites is further reduced due to the self-lubricating effect of graphite.At the later stage of the friction experiment,the contact surface area increased and a complete friction film was gradually formed,and making the surface more flat.The copper phase almost did not contact directly,which reduced the oxidation wear and adhesive wear.The area around the hard micro-convex points was filled with fine debris,which increased the contact area and reduced the abrasive wear,so mainly dominated by fatigue wear.After deformation and the accumulation of debris on the surface,a constant changing friction layer is formed,and its thickness and renewal speed are related to the wear rate of the composite material.