Worn Regions Research Articles

Single asperity nanoscale wear of carbides and matrix of wrought high carbon CoCrMo alloy

High carbon CoCrMo alloy has been widely used in orthopedic implants. Carbides improve the strength of the material and influence wear resistance. An atomic force microscopy (AFM)-based single asperity tribology method was developed to investigate the wear performance of individual carbides and base metal matrix of high carbon CoCrMo alloy. Sub-nano to nanoscale wear on single carbides was performed and quantified. Nanowear behavior of two types (Cr-rich and Mo-Rich) of carbides was stress-dependent (from 7.8 GPa to 9.8 GPa) and influenced by chemical composition. The lower wear depth of both Cr-rich and Mo-rich carbide compared to the matrix alloy shows higher wear resistance compared to metal matrix. Cr-rich carbides exhibit significantly smaller wear depth (p < 0.05) compared to Mo-rich carbides after nanowear at the same contact stress (9.8 GPa). The entire region of Cr-rich carbide shows high wear resistance without fracture after nanowear at 14.7 GPa. The AFM-based nanowear methods used are able to detect sub-nanometer wear depths on worn regions as small as 0.2 μm and provide abilities to study local nanometer wear processes in complex alloy systems.

Exploring computational techniques for simulating residual stresses for thin wall multi-joint hexagon configurations for a laser directed energy deposition process

Laser cladding is a directed energy deposition process and can lead to high residual stresses, which can compromise the quality of the specimen. As a result, it is crucial to accurately predict and investigate the residual stress distribution in cladded parts and understand the formation mechanisms. In this study, a thermo-mechanical metallurgical simulation model of the laser cladding process was developed for three different deposition sequences for a thin wall hexagon with inner junctions to investigate the formation of residual stress and distortion. The study was performed for single and multilayer scenarios. Two types of computational techniques, the detailed transient approach and the imposed thermal cycle approach, were performed and comparisons conducted. Consistent results were observed when comparing the resultant stress patterns for the single layer; subsequently, the imposed thermal cycle method was applied for the five-layer models. A preheat scenario is explored. This reduced the computational cost significantly, but the stress patterns were not similar. This indicates that building up worn regions at the top of a thin-walled component, such as a roll die, needs to be investigated further as unique issues have been highlighted. The differences between the implemented computational techniques are described as well as the advantages and disadvantages of each. Knowledge obtained from these case studies provides a foundation for efficient and rapid optimization of laser cladding processes, with the aim of minimizing residual stress in both simple and complex laser cladding structures.

Adhesive wear behaviour of surface modified bamboo filler reinforced polymer composite under different contact condition

ABSTRACT An experimental study has been conducted to evaluate the adhesive wear behavior of bamboo filler-reinforced polymer composite. Bamboo has the significant attention of investigators due to its benefits over human-made fibers. The bamboo fillers’ surface is treated by the alkali treatment process and the composite is prepared with a variation of micro bamboo particle support (i.e. 0, 2.5, 5, 7.5, 10, and 12.5%). The study of tribological characterization is explored for dry, wet, and heated sliding contact state using the pin-on-disk tribomachine counter to a stainless steel disc. Modified bamboo filler composite exhibited better interaction between filler and matrix, which improve the wear resistance capacity of the composite. From the experimental result it is found that specific wear rate and coefficient of friction is lesser in wet condition compared to dry and heated state conditions. Moreover, lower filler content offered enhanced wear performance related to the higher filler content composite. In case of dry sliding contact, debonding and plastic deformation are witnessed on the composite surfaces. However, the worn regions are minimal under wet state condition. Further, in case of the heated state, a thin layer created on the exterior of composite which safeguards the surface from further worn.

Wear Mechanism and Modeling of Tribological Behavior of Polycrystalline Diamond Tools When Cutting Ti6Al4V

Owing to its outstanding physical and mechanical properties, polycrystalline diamond (PCD) is ideal for cutting titanium alloys. However, the high temperature and stress caused by the interaction of tool surface and chip flow lead to different types of wear. This paper investigates the wear mechanisms of PCD tools in three different tribological regions: sticking zone, transition zone, and sliding zone, when machining titanium alloy Ti6Al4V. The tribological behavior of PCD tools in the wear processes were analyzed through both experiments and theoretical calculations. Analytical models of stresses and temperature distribution were developed and validated by turning experiments. PCD tools, consisting of diamond grains of different sizes: CTB002 (2 μm), CTB010 (10 μm), and CTM302 (2–30 μm), were used to cut Ti6Al4V at the normal cutting speed of 160 m/min and high cutting speed 240 m/min. It was found that adhesion, abrasion and diffusion dominated the wear process of PCD tools in different worn regions. Microscopic characters showed that the wear mechanisms were different in the three tribological regions, which was affected by the distribution of stresses and temperature. “Sticking” of workpiece material was obvious on the cutting edge, abrasion was severe in the transition zone, and adhesion was significant in the sliding zone. The shapes and morphological characters in different worn regions were affected by the stresses distribution and the types of PCD materials.

Dry Sliding Wear Behavior of Al2O3-TiC Ceramic Composites Added with Solid Lubricant CaF2 by Cold Pressing and Sintering

The solid lubricant CaF2 was added in Al2O3/TiC ceramic composites to reduce the friction coefficient and improve the wear resistance of the latter. The Al2O3/TiC/CaF2 composites were prepared by cold pressing and sintering. Experiments on friction and wear properties were conducted using a wear-and-tear machine. The wear surfaces were observed by scanning electron microscopy and energy-dispersive spectroscopy. The wear resistance properties and self-lubricating effect of samples were analyzed. Results show that the Al2O3/TiC/CaF2 composites have a lower friction coefficient than the Al2O3/TiC ceramic composites subjected to the same processes. The good wear resistance of the Al2O3/TiC/CaF2 ceramic composites stems from the low friction coefficient and wear rate. These parameters have a self-lubricating effect on wear surfaces. The mechanism of wear resistance is that smooth self-lubricating films form when CaF2 is applied on wear surfaces. CaF2 particles spread on friction pairs and fill micropits, thereby improving and smoothening worn regions.

A study on the effects of hardfacing thickness on wear characteristics of Stellite21 hardfaced STD61 hotworking tool steel at the elevated temperature

The aim of this paper is to investigate the effects of hardfacing thickness on wear characteristics of Stellite21 hardfaced STD 61 hotworking tool steel at the elevated temperature. Stellite21 super-alloy is deposited on the STD61 hot-working tool steel to fabricate the specimen using a laser-aided direct metal rapid tooling process. Several pin-on-disk wear experiments are carried out at temperatures beyond the full annealing temperature of the STD61. The hardness, the friction induced work hardening, the weight loss, the wear depth, and the wear width of each specimen are measured to quantitatively investigate the influence of the hardfacing thickness on the wear characteristics. The variation of friction characteristics of the worn region according to the hardfacing thickness is examined via the measurement of friction coefficients and the observation of morphologies of the worn regions for different specimens. From the results of these investigations, the proper thickness of the Stellite21 hardfaced layer on STD61 hot-working tool steel is estimated.

Electron microscopic evidence for a tribologically induced phase transformation as the origin of wear in diamond

Tribological testing of a coarse-grained diamond layer, deposited by plasma-enhanced chemical vapor deposition, was performed on a ring-on-ring tribometer with a diamond counterpart. The origin of the wear of diamond and of the low friction coefficient of 0.15 was studied by analyzing the microstructure of worn and unworn regions by transmission and scanning electron microscopy. In the worn regions, the formation of an amorphous carbon layer with a thickness below 100 nm is observed. Electron energy loss spectroscopy of the C-K ionization edge reveals the transition from sp3-hybridized C-atoms in crystalline diamond to a high fraction of sp2-hybridized C-atoms in the tribo-induced amorphous C-layer within a transition region of less than 5 nm thickness. The mechanically induced phase transformation from diamond to the amorphous phase is found to be highly anisotropic which is clearly seen at a grain boundary, where the thickness of the amorphous layer above the two differently oriented grains abruptly changes.

Laser cladding of rail steel with Co–Cr

Degradation and subsequent failure of rail tracks are commonly caused by rolling contact fatigue among other mechanisms of wear. Rail crossings are known to exhibit more of these failures due to increased localised traffic and environmental conditions. A high proportion of the costs associated with the repair of rail tracks was due to the rolling contact fatigue phenomenon. In order to mitigate these costs, laser cladding of worn regions has been proposed for the repair of used tracks in situ to limit the need for them to be replaced and for the preservice protection of newly rolled rails and cast crossings. A Co–Cr, Stellite 6, alloy is chosen to demonstrate repair and also surface coating/protection of R260 rail steel. Results showed that cladded Stellite 6 possessed improved hardness, good tribological performance and excellent workhardening ability when compared with rail steel. These demonstrate laser cladding as a viable solution for repair worn rail track.

The influence of femoral head surface roughness on the wear of ultrahigh molecular weight polyethylene sockets in cementless total hip replacement.

A theoretical relationship was recently proposed relating the wear behavior of polymetric bearing materials articulating against hard counterfaces.(1) This model attempts to predict the influence of surface roughness on wear. Laboratory-based studies have been used to establish the validity of these relationships, but their application to the clinical situation has not been investigated fully. Forty-two retrieved PCA hip joints have been assessed. The total wear volume was calculated from the penetration measured using the shadowgraph method, and roughness of the articulating surfaces was recorded using noncontacting profilometry. The roughness of the explanted femoral heads was observed to increase (median S(a) - 10. 35 nm worn region, 3.05 nm peripheral region), while that of the acetabular liner fell dramatically (median S(a) - 41 nm worn region, 212 nm unworn region). No evidence of a relationship between the topography of the worn regions of the femoral head and that of the acetabular liner could be found. Similarly, the strength of the association between the surface roughness and the clinical wear factor was considerably poorer than that achieved in laboratory experiments. A number of reasons for this observation are proposed. Most deleterious was considered to be the inability of the roughness parameters to describe the damaging features of the surface adequately. Uncertainty as to when the surface of the component degrades during its life serves to introduce further doubt as to the application of the wear models in the clinical environment. In conclusion, this study fails to provide clinical evidence to substantiate the relationship between surface finish and wear rate. The adoption of standardized measurement parameters and techniques would facilitate the direct comparison of joint types and the selection of the most advantageous materials.

Ceramic Roller Evaluation in Valve Train Tests

Motorized valve train tests at several cam shaft speeds were conducted on roller followers fabricated out of silicon nitride, sialon, and zirconia-toughened alumina, ceramics. The tests were conducted until the outer diameter of the ceramic roller spalled or 1000 hours were, successfully completed. Spalling damage occurred, on the reaction-bonded silicon nitride rollers that contained up to six percent porosity and on the sintered, zirconia-toughened alumina ceramics. The fully dense sintered or hot isostatically pressed silicon nitride and sialon ceramic rollers exhibited little wear. The worn regions of the cam lobes and the steel journal, shafts were analyzed for wear and related to the properties of the ceramic roller material. Presented at the 49th Annual Meeting in Pittsburgh, Pennsylvania May 1–5, 1994