• Ronkainen, Helena

Abstract

The amorphous hydrogenated carbon coatings (a-C:H)deposited byplasma-assisted chemical vapour deposition(PACVD), and tetrahedral amorphouscarbon coatings (ta-C)deposited by pulsed vacuum arc discharge wereevaluatedby scratch adhesion testing, load-carrying capacitytesting andtribological testing in unlubricated andlubricated sliding conditions. Thewear surfaces of thecoatings and counterparts were analyzed by optical andscanning electron microscopy, secondary ion massspectroscopy (SIMS), Augerelectron spectroscopy (AES),and micro-Raman spectroscopy.The adhesion ofthe a-C:H coating was improved bytitanium and titanium nitride intermediatelayers. Themost important parameters affecting the load-carryingcapacityof the a-C:H and ta-C coated systems proved tobe the hardness and Young smodulus of the substrate, andthe Young s modulus and internal stresses ofthe coating.In general, the ta-C coating was more wear-resistantcomparedto the a-C:H coating due to the higher hardnessof the coating. The wear ofthe counterparts slidingagainst the ta-C coating was one order of magnitudehigher compared to the a-C:H coating due to highersurface roughnesscombined with the high hardness of theta-C coating.For the a-C:Hcoatings, the increase in normal load andsliding velocity reduced thecoefficient of friction from0.42 to 0.1 against steel, and from 0.13 to 0.02againstalumina. The formation of graphite in the tribocontact ofthe a-C:Hcoating was observed by micro-Raman analyseswhen high normal loads andsliding velocities wereapplied causing high contact pressures and increasedcontact temperatures. The coefficient of friction of theta-C coating waslow and rather stable, in the range of0.10 to 0.19 in humid air (50%relative humidity). Somegraphitization on the wear surface of the ta-Ccoatingwas observed, but the amount of graphite detected wasrather smalldue to the stable sp3 structure of the ta-Ccoating. When the ta-C coatingswere tested in dry air(0% relative humidity) the coefficient of frictionincreased to the value of 0.71. However, by doping theta-C coating withhydrogen or methane, the coefficient offriction reduced as the hydrogencontent of the coatingincreased. These friction properties of a-C:H and ta-Ccoatings can be related to the friction properties ofgraphite, whichverifies the importance of graphiteformation and the availability ofhydrogen for lowfriction properties of the a-C:H and ta-C coatings.Inwater-lubricated tests, the ta-C coating showedexcellent tribologicalperformance with a dramatic dropin the coefficient of friction from 0.07 to0.03, but thea-C:H coating was susceptible to sliding wear inwater-lubricated conditions, which resulted in an earlyfailure of thecoating. The performance of the a-C:Hcoating in water could be improved byalloying thecoating with titanium.In oil-lubricated conditions, the a-C:Hand particularlyta-C coatings showed further improved tribologicalperformance compared to unlubricated sliding conditions.In oil-lubricatedconditions the tribofilm formationtypical for a-C:H andta-C coatings indry slidingconditions was prevented and the oil chemistry governsthetribocontact. The DLC coatings are not only suggestedfor use in dry slidingconditions, but also in boundarylubricated conditions to provide safeoperation indemanding conditions.

Topics

• Deposition
• surface
• amorphous
• Carbon
• nitride
• steel
• hardness
• Hydrogen
• titanium
• electron microscopy
• Raman spectroscopy
• selective ion monitoring
• atomic emission spectroscopy
• Auger electron spectroscopy
• coefficient of friction