Additive, lubricated wear behavior

The lubricated wear described above is squarely at odds with the behavior illustrated in Fig. 14-6 and with the wear-reducing action of 22% di-t-octyl disulfide in white oil reported by Dorinson and Broman [10] and shown in Table 11-6 (Chapter 11, Section 11.2.1). If Eqn 14-49 is a correct representation of additive action, it should be valid for both the reduction and the increase of wear by such action. To reduce wear, the first term on the right-hand side of the equation must control the overall rate and one way to do so is for the lump removal factor wear rate. But there is no physical necessity that q remains constant for all conditions of load, pressure, speed or state of lubrication. Since in physical terms the predominant effect of the lubricant is to inhibit the asperity adhesion process, it is not unanticipated that the average size of the transferred and detached particles as well as their number will be decreased by lubrication. It is to this latter type of mechanistic process that we must look for an explanation of why such parameters as contact pressure, rubbing speed and material properties affect the balance between the inhibition or promotion of wear by additive action and the transition from smooth lubricated wear to catastrophically damaging wear behavior such as scuffing. 

Much of the recent work on the friction and wear behavior of polyimides, with and without graphite fiber reinforcement and/or solid lubricant additives, has been conducted by Fusaro ( 1), who has recently reviewed the field (2). Reduction of friction and wear of polyimides by inclusion of graphite fibers, particularly at elevated temperature, is illustrated in a study by Fusaro and Sliney (3). In... 

Today, the role of ILs in the field of lubricants can be diverse instead of using them as a neat compound, they also can be used as an additive for other base oils, enhancing their friction, and wear behavior, but may also introduce novel properties such as conductivity. 

Similarly Liu and Li (1999) studied the influence of the addition of yttrium metallic powder as an additive in commercial lubricants on tribocorrosion resistance of AA6061 aluminum alloy and AIS1304 stainless steel grade. They compared the wear behavior of these two alloys in three mediums a vegetable oil, oil with sulfuric acid, and oil with sulfuric acid and yttrium. 

MWCNTs are the rolled graphene sheets, and thus they detain certain properties similar to that of graphene, which acts as a good solid lubricant. MWCNTs also exhibit lubricant characteristics, in addition to their superior mechanical properties. However, many studies have not reported the influence of MWCNTs on the wear behavior of UHMWPE. It is observed from the literature that the presence of MWCNTs could significantly reduce the wear of UHMWPE and thereby increase its longevity as a bearing surface. 

The work of Forbes and Battersby [46] is an integrated study of the relations among the chemical structures of the dialkyl phosphites, their adsorption on and reaction with iron, and their behavior in four-ball bench testing of lubricant additive effectiveness. The four-ball data in Table 11-17 for solutions of additive in white oil show that both the wear/load index (mean Hertz load) and the initial seizure load are critically responsive to concentration, with a strong effect when the concentration increases from 0.01 to 0.04 molal (0.031% to 0.124% P). The initial seizure load is an uncomplicated criterion with a straightforward interpretation, whereas the wear/load index is contrived, both in concept and performance. The low-load 50 minute wear data show inconsistencies in the influence of additives that have not been explained. 

Effective reactive pressureless sintering of SiC-TiB2 composites was recently reported by Blanc, Thevenot, and Treheux [155]. In addition they studied the tribological behavior using a pin on flat configuration (flat SiC, pin SiC-TiB2). In dry conditions the composites showed less wear resistance than monolithic SiC, however, with water as lubricant the opposite was the case. 

Improvement in wear is frequently cited as a motivation for current composite research in UHMWPE for orthopedic implants [43, 44, 47—51]. Thus far, the use of fillers alone has not proven effective in reducing the wear rate of UHMWPE by an order of magnitude, as has been observed with extensive radiation crossfinking. Whereas conventional UHMWPE may have been the state of the art when early research on UHMWPE composites was initiated, today UHMWPE matrix composites need to demonstrate superior properties when compared with unfilled radiation crosshnked materials. Because the tribology of UHMWPE in artificial joints is strongly dependent on the kinematics and lubricant, additional research is needed to fuUy characterize the biotribological behavior of UHMWPE micro- and nanocomposites for specific orthopedic bearing applications. 

Nanoparticles have received widespread acceptance as extreme pressure and anti-wear additives for liquid lubricants. If the nanoparticle deposits can act as a third body to help reduce asperity interactions and increase the load-bearing capacity of rubbing pairs (Rapoport et al, 2003) then action of such a third-body layer on tribocorrosion behavior will be of much interest. In addition, the possibility of using aqueous suspensions of nanoparticles as an alternative to oil-in-water emulsion is worth exploring. 

---