Abrasive wear

According to most studies dealing with wear, abrasion can be considered on one hand as micro-cutting, leading to material removal (grinding), and on the other hand as micro-ploughing, leading to plastic deformation and lower material removal (abrasive wear). Understanding various flowing transitions around an abrasive particle, under well-established conditions, makes it possible to better control these processes. The aim of the present study is to understand plastic deformation and failure local phenomena induced by an abrasive process. Experimental studies have been carried out on an abrasive process which consists in scratching a soft flat surface (AISI4140 steel) by mean of a turning tool fixed on the periphery of a disc. Regulating angular and feed speeds, successive scratches are expected to study phenomena generated by a single pass of the cutting tool. Scratch topography and forces are measured in order to highlight the influence of the depth of cut and the cutting speed on these parameters and on specific energy.

Use-wear analysis applied to two carnelian beads from Nahal Hemar Cave, southern Israel, and dated to
the Middle Pre-Pottery Neolithic B period, revealed a manufacturing procedure that corresponds to
genuine lapidary technologies of contemporary traditional societies. Based on ethnographic observations
combined with experiments in working carnelian, wear patterns were interpreted to be produced by a
multi-stage manufacturing sequence that includes abrasion against varying abrasion surfaces, drilling
probably with a splinter drill equipped into a rod and finally, tumbling. These beads are one of the
earliest examples of carnelian beads and thus represent a lapidary technology with roots from over 9000
years ago.

The wear resistance of plasma-sprayed ceramic coatings (Al 2 O 3 , Al 2 O 3 -13%TiO 2 , Cr 2 O 3 ) has been investigated through pin-on-disk and dry sand-steel wheel tests, has been correlated to microstructural and micromechanical characteristics (microhardness, fracture toughness) and has been compared to well-known platings (such as Cr electroplating and electroless Ni) and HVOF-sprayed cermets (WC-17%Co, WC-10%Co-4%Cr). Plasma-sprayed ceramics are hard but brittle: dry particles abrasion occurs through splats detachment. The toughest coating (Al 2 O 3 ) displays the highest wear resistance, which in fact overcomes HVOF-sprayed cermets and Cr electroplating, when a low number of wheel revolutions are considered. In pin-on-disk tests, no coating undergoes wear loss against the 100Cr6 ball, that possess lower hardness. Against the alumina ball, Al 2 O 3 and Al 2 O 3 -TiO 2 coatings show high wear rates and friction coefficients (due to chemical affinity), while Cr 2 O 3 possesses better wear resistance, lower friction coefficient and inflicts less wear on the counterpart. Cr 2 O 3 wear scar consists in plastically deformed splats and debris forming a quite adherent protective tribofilm.

In this work, cermet coatings were prepared by high-velocity oxygen-fuel (HVOF) technique using a Diamalloy 3007 powder. The influence of the spray parameters on corrosion, friction, and abrasive wear resistance was studied. The samples were obtained using the standard conditions (253 L/min of oxygen and 375 L/min of compressed air), higher oxygen flux (341 L/min), and higher carrier gas flux (500 L/min). The coatings were characterized using scanning electron microscopy (SEM), and x-ray diffraction (XRD). X-ray diffraction and SEM studies showed well-bounded coating/substrate interface, pores, metallic matrix, chromium oxides, carbides, and carbides dissolution into the matrix. In comparison with the standard condition, the sample prepared using higher oxygen flux showed the highest carbide dissolution because of the high temperature achieved in the spray process. When the carrier gas flux was increased, the sample showed denser structure because of the higher particle velocity. The friction and abrasive wear resistance of the coatings were studied using rubber wheel and ball-on-disk tests. All samples showed similar sliding and abrasive behavior, and all of them showed better performance than the aluminum alloy. The electrochemical behavior was evaluated using open-circuit potential (E OC ) measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization curves. The coating prepared with higher carrier gas flux showed the highest corrosion resistance in 3.5% NaCl solution and probably no pitting attack to the substrate occurred even after around 26 h of test. Tests performed for longer immersion times showed that the total impedance values significantly decreased (6 and 4 times) for samples sprayed using standard and higher oxygen flux, and no great change for sample sprayed using higher carrier gas flux was observed. The last sample presents a corrosion resistance around 200 times higher than the others.

The wear properties of A390 (Al-17Si-4.5Cu-0.5Mg, wt%) hypereutectic Al-Si alloy were compared to new alloys containing 6 and 10 wt% Mg. The wear behaviour was investigated using dry sand rubber wheel (DSRW) abrasive wear apparatus for these alloys in the as-cast condition and after T6 heat treatment. A transformation of hard and coarse primary Si particles to fine Mg 2 Si particles with inferior hardness occurs with increasing Mg content. The finer particle size together with increasing solid fraction of the primary phase was found to be key factors in improving the wear resistance of alloys with high Mg content. In addition, the eutectic matrix microstructure of alloys with high Mg content showed considerable changes, particularly in size and morphology of the eutectic silicon which significantly contributes to the wear resistance.

Ashless antiwear additives have begun to find applications in automotive and heavy industry applications as sludge formation is reduced leading to improved efficiency. In this study, six different ashless antiwear agents, including an ashless neutral dialkyl dithiophosphate (DDP-1), an acidic dialkyl dithiophosphate (DDP-2), an alkylated triphenyl phosphorothionate (A-TPPT), a butylated triphenyl phosphorothionate (B-TPPT), a triphenyl phosphorothionate (H-TPPT), and an amine phosphate (AP-1) were studied. Conditions of tribotesting were chosen to simulate harsh boundary lubrication where extreme loads are applied and the anti-wear additives are stressed. These test conditions can potentially separate out high performing additives from ones that provide marginal protection. The mechanism of wear in the best-case scenario with acidic dialkyl dithiophosphate was associated with the formation of well-developed tribofilms even under the harsh conditions. On the other hand the neutral dialkyl dithiophosphate exhibited severe abrasive wear and premature failure. The amine phosphate, while it has a functioning tribofilm, also exhibited weak adhesion of the tribofilm resulting in stick slip behavior and extensive polishing wear. All the phosphorothionates exhibited poor wear outcomes with the alkylated triphenyl phosphorothionate being the best and the protonated triphenyl phosphorothionate being the worst. The mechanism of wear in the phosphorothionates was associated with polishing wear in A-TPPT and B-TPPT where wear debris trapped between contacting surfaces removes the tribofilms that form on the surface and abrasive wear in the case of H-TPPT. Transmission electron microscopy of the harvested wear debris indicated the formation of amorphous debris with embedded nanocrystalline particles that were identified as Fe 3 O 4 in most of the cases. The wear protection afforded by the anti-wear additive can be correlated with the number of the nanocrystalline oxide particles embedded in the debris with the ones with smallest number of oxide particles exhibiting the best wear performance.

Tungsten carbide-cobalt (WC-Co) has been extensively employed as an abrasion/wear protective material. However, carbon loss (decarburization) of WC-Co powders during processing reduces the efficiency of the material against abrasive wear. This paper examines the efficiency of three consolidation processing techniques to obtain coatings and bulk samples from WC-Co powder. In this work, high velocity oxy-fuel (HVOF) spray forming and laser engineered net shaping (LENS Ò ) were applied to make WC-Co coatings; spark plasma sintering (SPS) was used to produce bulk WC-Co samples for comparison. The microstructures of two types of coatings and one bulk sample were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to recognize material modification. In addition, porosity and tribological properties of the WC-Co samples were tested. The ultra-microindentation technique was applied to measure the universal hardness and the elasto-plastic properties of the samples. Experiments using a tribometer (ball on disc configuration) under dry conditions have been performed in order to evaluate the friction and wear properties of the different systems.

The effects of nitriding, nitrocarburizing and shot peening on fatigue behavior of AISI D3 cold work tool steel were investigated. Five batches of fatigue specimens were manufactured according to the ASTM E466-96 standard. All specimens were quenched and tempered to a hardness of 50 HRC. One group of specimens was shot peened with an Almen intensity of 16 A, one group was nitrocarburized, one group was gas nitrided by single stage and another group was also nitrided by double stages. Rotating-bending fatigue tests were carried out. Our results showed that nitriding and nitrocarburizing treatments did not have a beneficial effect on the fatigue resistance of D3 tool steel. Fatigue test results revealed that shot peening increased the fatigue life of specimens by about 14%, while double stage nitriding decreased it by about 8%. Nitrocarburizing and single stage nitriding decreased the fatigue life of specimens by about 29% and 50%, respectively. Nitriding and nitrocarburizing treatments may improve abrasive-wear resistance of this material by increasing surface hardness but may promote the easy nucleation of a fatigue crack in the brittle surface layer and its unimpeded growth into the weaker substrate material. However, double stage nitriding is less damaging to the fatigue behavior of this steel. SEM and metallographic investigations indicated that a compound layer with porous structure was formed during nitriding and nitrocarburizing. So shot peening is the best treatment to improve the fatigue life of AISI D3 tool steel.

Thermal spray WC-based coatings are widely used in the aircraft industry mainly for their resistance to wear, reworking and rebuilding operations and repair of worn components on landing gear, hydraulic cylinders, actuators, propeller hub assemblies, gas turbine engines, and so on. The aircraft industry is also trying to use thermal spray technology to replace electroplating coatings such as hard chromium. In the present work, WC-Co coatings were built up on an AA 7050 aluminum alloy using high velocity oxygen fuel (HVOF) technology and a liquid nitrogen cooling prototype system. The influence of the spray parameters (standard conditions, W19S, increasing the oxygen flux, W19H, and also increasing the carrier gas flux, W19F) on corrosion, friction, and abrasive wear resistance were also studied. The coatings were characterized using optical (OM) and scanning electron (SEM) microscopy, and X-ray diffraction (XRD). The friction and abrasive wear resistance of the coatings were studied using Rubber Wheel and Ball on Disk tests. The electrochemical studies were conducted using open-circuit potential (E OC ) measurements and electrochemical impedance spectroscopy (EIS). Differences among coated samples were mainly related to the variation of the thermal spray parameters used during the spray process. No significant differences were observed in the wear resistance for the coatings studied, and all of them showed a wear rate around 10 times lower than that of the aluminum alloy. The results of mass loss and wear rate were interpreted considering different mechanisms. Comparing the different spray parameters, the oxygen flux (higher flame temperature) produced the sample which showed the highest corrosion resistance in aerated and unstirred 3.5% NaCl solution. Aluminum ions were detected on the surface almost immediately after the immersion of samples W19S and W19F in chloride solution, showing that the electrolyte reached the substrate and galvanic corrosion probably occurred. For sample W19H, aluminum ions were not detected even after 120 min of immersion in NaCl solution.