Influence of N additions on the structure, morphology, thermal stability and tribological properties of W-S-N coatings deposited by sputtering

Abstract

TMD coatings are a breakthrough in aerospace and automobile sector where low friction and low wear is required along with the coatings ability to withstand harsh and humid environments. The current study aims to systematically characterize the influence of N additions on structure, morphology, hardness, tribological behavior and thermal stability of WSN coatings deposited by sputtering. By varying the N2 flow into the deposition chamber, 4 coatings with N content ranging from 0 – 21.9 at. % were deposited. Highest S/W ratio of 1.5 was exhibited by reference WSN0 coating. Total film thicknesses along with Cr interlayer and gradient layer was in range of 2.1 – 2.4 µm. Reference WS2 coating had a crystalline structure, whereas with increasing N at. % content coatings exhibited broad XRD diffraction peaks as a result of the contribution of two different phases. Coating with the highest N concentration displayed an amorphous structure. Coatings were characterized tribologically against 100Cr6 steel ball in SRV tribometer at room temperature and 200ᵒC. Wear rate analysis showed that W-S-N coatings tested tribologically at high temperature performed better than the coatings tested at room temperature. Thermal stability was determined by annealing the coatings at 200ᵒC and 400ᵒC. No visible changes in the morphology and structure of the coatings were noticed with heat treatment.Conclusion: W-S-N coatings with progressive increase of N concentration were deposited from a WS2 target along with nitrogen gas flow in the chamber. The influence of N flow (sccm) on the properties of as deposited and heat-treated coatings was explored. • Increasing N flow from 0 to 20 sccm led to increase nitrogen content in coatings from 0 at. % to 22 at. %. The maximum S/W ratio (in case of WSN0) obtained was 1.45 for both as deposited and heat-treated coatings and then it decreased with N addition due to high affinity of W towards N instead of S. • Pure WSN0 coatings performed inadequately in terms of adhesion, hardness, and wear rate because of their loose and porous columnar morphology. Reference WSN0 coatings showed a crystalline structure, whereas with the increasing flow of nitrogen in the chamber coatings with higher concentration of N, such as WSN20 became X-ray amorphous. The presence of W-S-N phase in N rich coatings is not evident but W2N phase could be indexed in the XRD diffractograms. N incorporation in the lattice structure with low S/W ratio have caused the shift in peak for N rich coatings. • The hardness of coatings has evidently increased as the morphology became compact with nitrogen addition in the coatings with WSN12.5 being the highest in value. The coatings that were annealed displayed better hardness results in comparison to the as deposited coatings. This hardness increase of coatings during annealing is due to the increase of their crystallinity. W-S-N sputtered coatings displayed excellent sliding properties in comparison to literature with good coating adhesion and low wear rates. • The tribological results obtained with low S/W ratio along with nitrogen doping were quite impressive and current study appeared to be the successful beginner step in upscaling the industrial applications for systems sliding in high temperature atmospheres.

A aplicação de revestimentos do tipo TMDs no setor aeroespacial e automóvel têm permitido avanços significativos em componentes onde baixo atrito e baixo desgaste são necessários, juntamente com a capacidade dos revestimentos de resistir a ambientes hostis e húmidos. O presente estudo visa caracterizar sistematicamente a influência da adição de N na estrutura, morfologia, dureza, resistência ao desgaste e estabilidade térmica de revestimentos do sistema WSN depositados por pulverização catódica. Variando o fluxo de N2 na câmara de deposição, 4 revestimentos com teor de N (entre 0 e 21,9 at. %) foram depositados. O revestimento de referência A razão S/ W mais alta de WSN0 apresentou a razão mais alta de S/W. A espessura total dos filmes juntamente com a intercamada de Cr e a camada de gradiente encontra-se entre 2.1 – 2.4 µm. O revestimento de referência apresenta uma estrutura cristalina. O aumento do teor de N nos revestimentos resulta num alargamento dos picos de difração devido à contribuição de duas novas fases. O revestimento com maior concentração de N apresenta uma estrutura amorfa. Os revestimentos foram caracterizados tribologicamente num equipamento SRV contra uma bola de aço 100Cr6 à temperatura ambiente e a 200ᵒC. Os revestimentos W-S-N testados tribologicamente aalta temperatura apresentaram um melhor desempenho que os revestimentos testados à temperatura ambiente. A estabilidade térmica dos revestimentos foi avaliada a 2 temperaturas distintas (200 and 400 0C), onde não se observaram alterações estruturais e morfológicas.Conclusion:W-S-N coatings with progressive increase of N concentration were deposited from a WS2 target along with nitrogen gas flow in the chamber. The influence of N flow (sccm) on the properties of as deposited and heat-treated coatings was explored. • Increasing N flow from 0 to 20 sccm led to increase nitrogen content in coatings from 0 at. % to 22 at. %. The maximum S/W ratio (in case of WSN0) obtained was 1.45 for both as deposited and heat-treated coatings and then it decreased with N addition due to high affinity of W towards N instead of S. • Pure WSN0 coatings performed inadequately in terms of adhesion, hardness, and wear rate because of their loose and porous columnar morphology. Reference WSN0 coatings showed a crystalline structure, whereas with the increasing flow of nitrogen in the chamber coatings with higher concentration of N, such as WSN20 became X-ray amorphous. The presence of W-S-N phase in N rich coatings is not evident but W2N phase could be indexed in the XRD diffractograms. N incorporation in the lattice structure with low S/W ratio have caused the shift in peak for N rich coatings. • The hardness of coatings has evidently increased as the morphology became compact with nitrogen addition in the coatings with WSN12.5 being the highest in value. The coatings that were annealed displayed better hardness results in comparison to the as deposited coatings. This hardness increase of coatings during annealing is due to the increase of their crystallinity. W-S-N sputtered coatings displayed excellent sliding properties in comparison to literature with good coating adhesion and low wear rates. • The tribological results obtained with low S/W ratio along with nitrogen doping were quite impressive and current study appeared to be the successful beginner step in upscaling the industrial applications for systems sliding in high temperature atmospheres.