The structural responses of multifunctional materials to mechanical forces are complex because they are composed of the mechanical properties of their constituent materials. The process gets more complicated during the wear phenomenon where several mechanical, chemical, and thermal interactions occur within the multifunctional material, antagonist, and surrounding fluid 38,39. Based on the aforementioned results, the mechanical interaction between the biocomposites and antagonist stainless-steel balls in the modified Fusayama solution bath was further discussed in this section. Methacrylate-based biocomposites are inherently brittle and abrasive wear is the dominating mode against the stainless-steel antagonist. However, once the fibres are added, the material structure and the response to the external loads are more complex. The rotary studies in this test were conducted to confirm the reciprocating test results for specific wear rate and wear phenomenon.
The variation of CoF with time and the average CoF depends on the shape of the contact area, applied normal load, lubricating conditions, and shape and size of debris particles. In the current study, the fibre-to-filler weight fraction had a significant influence on the average CoF, whereas the fibre AR seemed to have minimal impact. The average CoF increases as the fibre-to-filler weight fraction increases. The finding is consistent with the earlier studies 20. This could be due to all three fibre lengths being smaller than the critical fibre length, which can reduce fibre-resin interfacial bond strength. For Group A, CoF rises rapidly and had a lower steady-state value with a shorter transient period. With the replacement of fillers with fibres for Groups B to E, the material structure changed, and the time taken to reach the steady state become longer. The transient period of Group-A is short wherein the CoF rises rapidly and had a lower steady-state value. Higher CoF is seen with the addition of glass fibres from 5 to 15 wt%. Also, the variation of fibre AR from 50 to 100 had minimal effect on the average CoF for the tested Groups. The rotary wear test shows a similar trend of CoF as the reciprocating wear test as the fibre-to-filler weight fraction increases. Since the rotary wear test creates a centrifugal force that pushes debris particles away from the centre of the circle wearing path, the quantity of debris particles trapped within the wear track is reduced. As a result, the effect of the three-body abrasion wear mechanism on the biomaterial is lower for the rotary wear test. Therefore, the average CoF for the rotary wear test is relatively lesser compared to the reciprocating wear test for the same fibre-to-filler weight fraction and fibre AR biocomposite. This phenomenon also affects the specific wear rate; rotary wear-tested biocomposites exhibit a lesser wear rate compared to reciprocating wear test.
Fatigue wear generally originates from stress concentrations such as voids, scratches and dissimilar material interfaces 40. Cracks are generated either on the surface or subsurface which could grow and dislocate the local material at the microscopic/macroscopic scale in the form of debris 19. All the tested Groups demonstrated wear grooves (ploughing) along with debris where the soft resin and reinforcements are removed by the asperities of the antagonist as seen in SEM images in Figs. 4–8. Group-A had several partially detached flake-shaped wear particles (Fig. 4, arrowed) which are developed due to shearing of the softer polymer caused by the interaction of normal and tangential loads on the surface 41 and/or subsurface crack propagation [20]. For Groups B-J, the SEM images (Fig. 5–7) revealed that the resin matrix removal during the wear process resulted in glass fibres being exposed to the antagonist, which were eventually fractured and detached from the resin matrix. Wear tracks of all tested Groups show the matrix cracking along with debris and filler clusters. Composites in Group B show matrix wear around the fibre, exposure of fibres on the surface, fibre thinning and fibre breakage (Fig. 7). Fibre debonding is evident in the wear tracks of Groups C, D, F, H, I and J. Fibre removal/plucking is seen in all Groups which creates a cavity which in turn increases CoF and higher material removal. The wear track surfaces also contain flakes (a general characteristic of fatigue wear) and debris-filled cavities (Fig. 7–9). The flakes act as concealment for the underneath resin matrix and protect the resin matrix from tangential abrasion. For Groups I-J with 15 wt% fibres, a higher number of partial flakes and compressed debris is seen which reduced the amount of wear on the surface. The SEM images show no fibre residue on any of the sample surfaces. This is due to such residue being washed away by the modified Fusayama solution during both the reciprocating and rotary wearing process.
SEM analysis of the wear surfaces revealed to be abrasive and fatigue wear, where the material on the surface is removed or displaced in the micro/nanoscale, increasing the surface roughness. For example, in the case of dentistry, this high surface roughness of the occlusal surface of a tooth restoration creates microcavities that promote bacterial colonies [3] and forms plaque, reducing the structural integrity of the restoration 42. The measured mean post-wear surface roughness values of the tested Groups were similar to those for amalgam 43 and particulate-based biocomposites 11,44. Post-wear surface roughness in the current study demonstrated an increase for 5 and 10 wt% fibre loading for all three fibre lengths and a reduction for 15 wt% loadings. The reduction for 15 wt% loadings can be attributed to the presence of compressed debris masking the wear grooves and cavities of plucked fibres which were lower for the 5 and 10 wt% Groups, but higher for the 15 wt% Groups as seen in the SEM images in Fig. 6–9. The 5 and 10 wt% Groups which had a higher quantity of glass particles in the composite are likely to prevent the deposition of debris in the wear grooves.
Based on the SEM analysis of the wear surfaces, the wear mechanism of the tested short glass fibre reinforced bio composites could be summarised as (i) removal of matrix material due to abrasion by the antagonist; (ii) weakening of the interfacial bond between the resin and reinforcement (glass particles and fibres) 45; (iii) dislodging of filler particles; (iv) exposure of glass fibres due to the detachment of surrounding filler and matrix; and (v) fibre thinning and/or breakage (cantilever action) due to further abrasion. The finding is in agreement with the earlier studies of Zhang et al. and Ozturk and Ozturk 46,47.
Although the qualitative SEM analysis of the wear track surface allows us to understand the effects of fibre-filler fraction and wear on the evolution of microstructure, it does not provide comprehensive information to understand the wear mechanism, the subsurface damage of the resin matrix, and correlation of the wear track microstructure with the specific wear rate. Hence, the wear cross-section of the tested biocomposite Groups was also observed via SEM and they show the appearance of fibre pull-out cavities, microcracks, and fibre-resin interface failure. These provide additional information about the path length and direction of microcrack propagation within the biocomposite matrix, fibre-resin interfacial adhesive and interlocking properties. Group N has the highest fibre-to-filler weight fraction, but it has a lower specific wear rate compared to Group M. The SEM images show that Group N has a shorter microcrack path length compared to the Group M. Severe fatigue fracture takes place on the top wear surface. Partial plastic deformation occurs under this surface. The degree of plastic deformation decreases with depth. The vertical crack elongation may involve crack initiation, crack growth from top to bottom, and crack termination by fibres and fillers. The accumulation and compression of debris in the microcrack voids cause the microcracks to expand in all directions. Since the propagation direction and the path length of the microcrack is being limited, Group N has a lower wear volume compared to Group M. The SEM findings consistent with the specific wear rate results.
Past investigators have observed a direct correlation between the specific wear rate and fibre weight fraction 20 for glass fibre reinforced composites that did not contain particulate reinforcements. However, for the present composites which contained particulate reinforcements in the range of 40 to 55 wt%, the wear rate increased from 5 to 10 wt% of fibres but decreased for 15 wt% of fibres. Several researchers 39,48,49 have proposed a “protection hypothesis” to justify a reduction in specific wear rate with an increase in fibre fraction. That is, the weak resin-rich area around the fibres (due to mismatch in the geometries) is more rapidly abraded, creating a void surrounding the fibre. The displaced 0.7 µm filler particles get embedded in the micro-pores on the wear surface as well as in the voids surrounding the glass fibres. The compressed debris increases the stiffness of the material locally and hence reduces the wear rate. Higher wt% of fibres are also known to increase crack bridging and crack deflection capabilities, thereby enhancing the fracture toughness and fatigue performance 50. The key contributor to the low specific wear rate for Groups H-J is likely to be high fracture toughness, as reported by Heintze et al. 51 and Kim and Watts 4.
The difference in the material hardness between the biocomposite components and the steel ball also is an influencing parameter in this study. The hardness of stainless-steel balls is 190 HV (3 Moh) 52 whereas the hardness of S-Glass fibre is 900 HV (6.5 Moh) 53. Since the S-Glass fibre is harder than the steel ball, the glass fibre in the biocomposite could abrade the steel ball and the degree of abrasion increases as the fibre-to-filler weight fraction increases. This can be seen as the shallower and wider wear trajectory observed on the biocomposite (Figs. 1(a-b)) and the enlargement of the spalling pits on the steel ball (Fig. 10) as the fibre-to-filler weight fraction increases. Hence, the fibre-to-filler fraction in the biocomposites is the main factor affecting the abrasion and fatigue mechanism of the wear test compared to fibre AR.