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Improved multicomponent coatings for tribological applications

Self lubricating coatings have to face two major requirements: high hardness for sufficient wear resistance and local low shear strength to provide good sliding behaviour towards counterbodies. There is a high variety of possible solid lubricant materials such as sulphides, fluorides, oxides, glasses and many others. The choice for further investigations was on alloys of 70TiC-Fe20Cr-5..7Mo2C (TiC for hardness, Mo2C for lubricant and Fe20Cr for toughening), FeCSi+4,5CaF2, FeCSi+7..14FenOm, Cr2O3+6..13CaF2 and C2O3-xTiO2-yCaF2.

The production was carried out by using methods such as SHS processing and mechanical mixture. The SHS process could be improved by using certain powder compositions. The starting powder consisted of Ti and carbon and Fe20Cr and Ti, Cr and CaF2 respectively. The other materials were produced by mechanical mixtures.

A model describing heating and acceleration of multicomponent materials in Argon and Nitrogen plasmas was created. Results show, that particles of CaF2 and Fe3O4 would instantly evaporate if injected directly at the nozzle's exit. Thus it is advantageous to inject matrix and solid lubricants separately. The results also show, that particles of different density should be injected into the plasma with different powder velocities to ensure best penetration into viscous plasma.

Coatings were produced by Detonation-Gun Spraying (D-Gun), Atmospheric Plasma Spraying (APS) and High-Velocity Plasma Spraying (HVPS). Parameters were varied and adapted to the various materials. Coatings were metallographically evaluated, phase analysis was conducted by X-Ray analysis and EDX and microhardness was determined. Augerspectroscopy was used to detect lubricant inclusions (C and CaF2) in the coatings. Results show, that beside a few all materials can be sufficiently processed to coatings by all processes with minor or major advantages. Spraying process has significant influence on formation of coating morphology and phase composition as well as on oxide content.

Characterisation work in order to determine wear and friction behaviour was carried out with a Ball-On-Disk testing apparatus. The experimental results are the variation of the coefficient of friction (COF) versus the numbers of rotation cycles. These results show only for few samples reasonable low coefficients of friction (around 0,4). At a first glance the results are apparently not indicating the expected self-lubricant behaviour tested at room temperature. Better behaviour can be expected at elevated temperatures.

Reported by

Rhein-Westfälische Technische Hochschule Aachen
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