Traugott E. Fischer

Microstructure, hardness and toughness of nanostructured and conventional WC-Co composites

Abstract The microstructure and mechanical properties of nanograin-sized WC-Co composites were investigated and compared with those of conventional cermets. The dislocation density in the nanometer-sized WC crystals is lower than in the conventional ones, and no inclusions are observed in them. Nanostructured composites have higher tungsten content in the binder phase and a higher FCC HCP ratio of the cobalt. An amorphous phase is observed in the binder phase of the nanostructured samples. Hardness and surface toughness were investigated by performing Palmqvist indentations at loads from 0.025 to 40 Kg. The hardness increases with decreasing binder mean free path of dislocation in the binder phase. The high hardness of nanostructured cemented carbides results not only from the ultrafine microstructure, but also from alloy strengthening of the binder phase itself. The variations of hardness with load suggest that the finer grade conventional carbides have higher microfracture strength, and the nanostructured WC-Co composites are superior to the conventional ones in this respect. Bulkfracture toughness is related to cracks developing through the phases of the material. Palmqvist indentation toughness measurements show that the toughness decreases with increasing hardness in conventional composites, whereas the increase of hardness in nano-structured composites does not further reduce their bulk fracture toughness. This implies that different dominant toughening mechanisms exist in the conventional and nanostructured composites.

Abrasion resistance of nanostructured and conventional cemented carbides

Abstract The abrasion resistance of nanostructured WCCo composites, synthesized by a novel spray conversion method, is determined and compared with that of conventional materials. Scratching by diamond indenter and abrasion by hard (diamond), soft (zirconia) and intermediate (SiC) abrasives was investigated. The size of the scratch formed by the diamond is simply related to the hardness of the composite. Plastic deformation, fracture and fragmentation of the WC grains increase with their size. Nanoscale composites show purely ductile scratch formation. Nanocomposites possess an abrasion resistance approximately double that of the most resistant conventional material: this is a higher gain than the increase in hardness which is at most 23%. This large gain is due to a specific grain size effect on abrasion resistance in the case of diamond and SiC abrasive and to a very rapid increase of abrasion resistance with hardness in the case of the softer (SiC and ZrO2) abrasives. The observation of the abraded surfaces of conventional composites reproduced the known mechanisms: plastic deformation and fracture of WC grains by hard abrasives; removal of binder phase and fall-out of WC by soft abrasives. Magnetic fields from the ferromagnetic Co prevent the observation of abrasion mechanisms in the very fine-structured nanocomposites.

Sliding wear of conventional and nanostructured cemented carbides

Abstract The sliding wear mechanisms of cemented carbide and the effects of the microstructure scale on the wear resistance were investigated by performing a series of unlubricated sliding wear tests in air with pins of WCCo composites sliding against silicon nitride disks. In the first approximation, the wear rate is proportional to the hardness with a wear coefficient k = 6.9 × 10 −6 for all materials. In the conventional cermets, the wear coefficient k also depends on the grain size; materials with smaller WC grains exhibit a smaller wear resistance. This reduction, however, does not extend to the nanostructured materials which exhibit the above value for k : their wear resistance is higher than that of conventional cermets in proportion to their hardness. The data can also be expressed in terms of cobalt content; the lower the cobalt content, the lower the wear; but two different such dependencies exist, one for the conventional and one for the nanostructured materials with lower wear. The sliding wear of WCCo composites occurs on a very small scale: the worn surfaces show no evidence of fracture or plastic deformation. This wear behavior is explained by the hexagonal structure and the anisotropic mechanical behavior of the WC grains that are capable of shear in a limited number of planes but are not capable of triaxial deformation. The higher wear resistance of the nanostructured composites is related to their hardness which decreases the real area of contact.

The effects of fuel chemistry and feedstock powder structure on the mechanical and tribological properties of HVOF thermal-sprayed WC–Co coatings with very fine structures

Abstract We have deposited a series of WC–Co coatings by the high velocity oxy fuel (HVOF) process, using three different powders, and different spray conditions. The powders are a nanocrystalline (Nanocarb), a near-nanostructured powder (Infralloy) containing a proprietary additive aimed at retarding grain growth, and multimodal (mixed micro and nano) powder (Nanomyte). HVOF spray conditions were stoichiometric, fuel rich and oxygen rich. ‘In-flight’ feedstock powder temperature and velocity were measured. The hardness and toughness of the coatings are found to depend on WC-binder adhesion and adhesion between splats. High flame temperatures increase WC-binder adhesion but increase decarburization. The latter is found to decrease adhesion between the splats. Decarburization is most pronounced for nanostructured powders because of their high specific surface. The additive in Infralloy decreases adhesion between WC grains and binder, but it also reduces decarburization. The wear resistance of the coatings increases with hardness and decreases with increasing decarburization. Sliding wear occurs by a attrition of the WC grains and the lifting of entire splats; abrasive wear occurs by ductile cutting, grain loss and lifting of splats; the low wear rate in sliding leads to splat-boundary weakening by fatigue. The effect of decarburization predominates in sliding wear and is less pronounced in abrasion; the high abrasive wear removes material before fatigue becomes important. The coating deposited at high temperature, from the multimodal powder Nanomyte, presents outstanding sliding and abrasive wear resistance but inflicts large wear on the opposing silicon nitride surface in sliding. Coatings deposited with the near nanostructured powder containing an additive present high sliding wear resistance, independent of the deposition parameters, and cause low wear of the opposing silicon nitride. Coatings deposited with spray-dried nanostructured powders offer comparatively low wear resistance, in agreement with previous reports.

The effects of oxygen and humidity on friction and wear of diamond-like carbon films

Diamond-like carbon (DLC) films were prepared on Si(100) wafers by plasma-assisted chemical vapor deposition at a pressure of 900 mTorr. Their wear rates and friction coefficients against a silicon nitride ball were measured in a pin-on-disk tribometer in argon and air with varying relative humidities. In 50% relative humidity, the measured wear rates of the ball and DLC were of the order of 10−8 mm3 N−1 m−1 and 10−7 mm3 N−1 m−1 respectively. In dry argon, dry air and 100% humid air, the wear rates of DLC were 10−9, 10−9 and 10−8 mm3 N−1 m−1, but that of the ball was below the detection limit. The measured friction coefficients were 0.06 in dry argon, 0.08 in 50% humid air and 0.09 in 100% humid argon, and around 0.2 in 50% humid argon, dry and 100% in humid air. In dry argon, the contact area of the ball was covered with material transferred from the DLC film during sliding; the low friction coefficient and wear rate measured in dry argon are attributed to this material. In dry and humid air, surface layers of DLC were oxidized by a tribochemical reaction forming a CO bond. They covered the contact area of both the DLC film and the ball. This film increased friction coefficients, but it acted as a protective coating when its thickness was sufficient to prevent direct contact of the DLC film against the ball in 100% humid air.

Genesis and role of wear debris in sliding wear of ceramics

Abstract We survey the formation, appearance and properties of wear debris in ceramics and their influence on the wear of these materials. These differ from those of metals, in accord with the different mechanical and chemical properties of ceramics. Ceramics, in contrast to metals, do not form hard and cohesive mechanically alloyed surface layers. The formation of wear debris is very sensitive to the environment (humidity), it occurs mostly by fracture on different scales: microfracture at low loads, grain boundary fatigue at intermediate loads and macroscopic fracture at high loads. The wear debris that remain in the wear track are ground to a very fine powder by continued rubbing. In dry ambient, this powder has low mechanical strength and has very little influence on wear. In humid ambient, the phenomena depend on the material. Tribochemical reactions of non-oxide ceramics form relatively large amounts of compact hydrated oxide. On oxide ceramics such as alumina and silica, interaction with water vapor has been observed to create thin hydroxide layers, which can act as lubricants, as observed on alumina. The tribochemical layers often form rolls on the surface. These rolls do not act as ‘rolling bearings’ and do not reduce friction or wear. Ambient humidity causes adhesion between the wear debris which are compacted into layers that have sufficient cohesion to reduce wear by distributing the contact stresses. In water and some aqueous solutions, silicon nitride and silicon carbide dissolve in water and do not form wear debris.

Comparison of HVOF and plasma-sprayed alumina/titania coatings—microstructure, mechanical properties and abrasion behavior

Abstract We have evaluated the microstructure, mechanical properties and abrasion wear resistance of alumina/titania ceramic coatings deposited with nano- and micro-structured powders by high-velocity oxygen fuel (HVOF) and plasma spray (PS) processes. The deposition guns have a strong influence on the mechanical properties and abrasive wear resistance of the coatings, but the powders do not. The coatings deposited by HVOF are significantly harder and tougher, and their abrasion resistance is two–three-fold higher. Plastic microcutting plays the predominant role in abrasion wear of the coating deposited by HVOF. A combination of brittleness and porosity results in fracture that dominates the abrasion wear of plasma-sprayed coatings. The abrasion resistance measured follows an Evans–Marshall equation modified to account for the effects of porosity.

Sliding and abrasive wear resistance of thermal-sprayed WC-CO coatings

We studied the resistance of the coatings to abrasive and unlubricated sliding wear of 40 WC/Co coatings applied by high velocity oxygen fuel (HVOF), high-energy plasma spray (HEPS), and high velocity plasma spray (HVPS), using commercial and nanostructured experimental powders. The hardness of the coatings varies from 3 to 13 GPA, which is much lower than that of sintered samples (10 to 23 GPA) because of the porosity of the coatings. Phase analysis by x-ray diffraction revealed various amounts of decarburization in the coatings, some of which contain WC, W2C, W, and η phase. The abrasive and sliding wear resistance is limited by the hardness of the samples. For a given hardness, the wear resistance is lowered by decarburization, which produces a hard but brittle phase. Nanocarb powders have the shape of thin-walled hollow spheres that heat up rapidly in the gun and are more prone to decarburization than commercial materials. The work shows that, in order to obtain the performance of nanostructured coatings, the powder and spray techniques must be modified.

TRIBOLOGY OF DIAMOND-LIKE CARBON SLIDING AGAINST ITSELF, SILICON NITRIDE, AND STEEL

Diamond-like carbon (DLC) films were deposited on (100) silicon wafers and silicon nitride balls by RF plasma-assisted chemical vapor deposition at a pressure of 700 mTorr and a substrate temperature of 360 K. The friction coefficient and the wear rates were measured using a pin-on-disk tribometer in 40% humid and dry air. Friction coefficients are near 0.05 in all cases measured. In dry air, the wear of silicon nitride and steel against DLC is below measurement capability because of a protecting DLC transfer layer, and wear of DLC is 2.5 ± 10 −8 mm 3 /Nm against silicon nitride and 6.5 ± 10 −9 mm 3 /Nm against steel. In humid air, the DLC transfer layer does not adhere to the solids, and wear of both bodies is larger. Unmeasurable wear is obtained when DLC slides against itself in humid air; the wear rate is 5 ± 10 −9 mm 3 /Nm in dry air. These results are interpreted in terms of the properties of a friction-induced transformation of the surface layer of DLC.

Multimodal powders : A new class of feedstock material for thermal spraying of hard coatings

We are developing a new class of High Velocity Oxy-Fuel (HVOF) thermal spray feedstock powders, which consist of aggregates of hard ceramic particles that are either mixed or coated with a more readily fusible nanophase binder. Thus, during thermal spraying, the nanostructured material undergoes rapid melting while the aggregated material is heated but not necessarily melted. A dense coating is formed when the molten nano-material fills the available pore spaces between the heated and softened aggregates, providing a strong and tough matrix for the spray deposited material. Such multimodal coatings combine moderate hardness with exceptional abrasive wear resistance. Characteristics of the multimodal feed powders, along with the structure and properties of the resulting coatings, are described in this paper.