Examination of the material removal mechanisms during the lapping process of advanced ceramic rolling elements

Two types of HIPed Si3N4 bearing ball blanks with different surface hardness and fracture toughness were lapped under various loads, speeds, and lubricants using a novel eccentric lapping machine. The lapped surfaces were examined by optical microscope and SEM. The experimental results show that the material removal rate for type I ball blanks were 3-4-fold of type 2 in most cases. Different lapping fluids affected the material removal rate at lower lapping loads, but they had much less influence on the material removal rate at higher lapping loads. The SEM micrographs reveal that the grain pullout prevailed on the lapped surface of type I ball blanks, and the surface of type 2 featured bulk material removal by microcracking. Under extreme high lapping load, surface cracks and damages were found, and SEM with EDX disclosed steel from the lapping plate had transferred to the ceramic ball surface. The preliminary conclusion is that the material removal mechanism during the lapping process of silicon nitride balls using this eccentric lapping machine is mainly mechanical abrasive wear. Lawn and Wilshaw's indentation model on brittle materials is used to explain the difference in material removal rate for the two types of ball blanks

Development of superlattice CrNNbN coatings for joint replacements deposited by High Power Impulse Magnetron Sputtering

The demand for reliable coating on medical implants is ever growing. In this research, enhanced performance of medical implants was achieved by a CrN/NbN coating utilising nanoscale multilayer/superlattice structure. The advantages of the novel High Power Impulse Magnetron Sputtering technology, namely its unique highly ionised plasma were exploited to deposit dense and strongly adherent coatings on Co-Cr implants. TEM analyses revealed coating superlattice structure with bi-layer thickness of 3.5 nm. CrN/NbN deposited on Co-Cr samples showed exceptionally high adhesion, critical load values of LC2= 50 N in scratch adhesion tests. Nanoindentation tests showed high hardness of 34 GPa and Young's modulus of 447 GPa. Low coefficient of friction (µ) 0.49 and coating wear coefficient (KC) = 4.94 x 10-16 m3N-1m-1 were recorded in dry sliding tests. Metal ion release studies showed a reduction in Co, Cr and Mo release at physiological and elevated temperatures, (70 oC) to almost undetectable levels (<1 ppb). Rotating beam fatigue testing showed a significant increase in fatigue strength from 349±59 MPa (uncoated) to 539±59 MPa (coated). In vitro biological testing has been performed in order to assess the safety of the coating in biological environment, cytotoxicity, genotoxicity and sensitisation testing have been performed, all showing no adverse effects. Keywords: Orthopaedic implant, High Power Impulse Magnetron Sputtering, Superlattice coating, Corrosion, Biocompatibility

The background for the use of hartmetals and MMCs based on Niobium Carbide (NbC) as cutting tools and for wear resistant tribosystems

In this present study, the mechanical properties (strength, hardness, moduli) and the drysliding properties of stoichiometric and sub-stoichiometric NbC were compared. Microhardnessand elastic properties of NbC depend from the C/Nb ratio, because the binaryphase diagram Nb-C shows a region of homogeneity of NbCx of 0,72≤ x ≤1.0. At RT, hardmetals of stoichiometric NbC have an elastic modulus E of ~440 GPa, those of substochiometricNbC0,88 an E of 405 GPa. The hot hardness of sub-stoichiometric NbC is above600°C higher than of WC. The dry sliding wear resistance (0,1-7/10 m/s) of the presentFe3Al-NbC0,94 with ~61 vol.-% NbC as hard phase was close to those known of NbC-basedhard metals. No grain pull-outs or fragmentations of the NbC grains were seen in the weartracks of the Fe3Al-NbC composite (MMC), as a metallurgical interphase was formedbetween matrix and NbC grains. Stoichiometric and sub-stoichiometric niobium carbideshave at RT and 400°C under dry sliding a prone intrinsic wear resistance more or less independentfrom sliding speed, either as hardmetal or as hard phase in metal matrix composite,associated with an exceptional high load carrying capacity

Comparison of self-mated hardmetal coatings under dry sliding conditions up to 600 degrees C

Hardmetal coatings prepared by high velocity oxy-fuel (HVOF) spraying represent an advanced solution for surface protection against wear. In the current systematic study the high-temperature oxidation and unidirectional sliding wear in dry and lubricated conditions were studied. Results for a series of experiments on self-mated pairs in dry conditions as part of that work are described in this paper. Coatings with nominal compositions WC-10%Co4%Cr, WC-(W,Cr)(2)C-7%Ni, Cr3C2-25%NiCr, (Ti,Mo)(C,N)-29%Ni and (Ti,Mo)(C,N)-29%Co were prepared with an ethylene-fuel led DJH 2700 HVOF spray gun. Electrolytic hard chromium (EHC) coatings and bulk (Ti,Mo)(C,N)-15%NiMo (TM10) hardmetal specimens were studied for comparison. The wear behaviour was investigated at room temperature, 400 and 600 degrees C. For the coatings sliding speeds were varied in the range 0.1-1m/s for a wear distance of 5000 m and a normal force of 10 N. In some cases the WC- and (Ti,Mo)(C,N)-based coatings showed total wear rates (sum of wear rates of the rotating and stationary samples) of less than 10(-6) mm(3)/Nm, i.e., comparable to values typically measured under mixed/boundary conditions. Coefficients of friction above 0.4 were found for all test conditions. The P x V values as an engineering parameter for coating application are discussed. The microstructures and the sliding wear behaviour of the (Ti,Mo)(C, N)-based coatings and the (Ti,Mo)(C,N)-15%NiMo hardmetal are compared