Amorphous silicon carbide (a-SiC) films are promising solution for functional coatings intended for harsh environment due to their superior combination of physical and chemical properties and high temperature stability. However, the structural applications are limited by its brittleness. The possible solution may be an introduction of nitrogen atoms into the SiC structure. The effect of structure and composition on tribo-mechanical properties of magnetron-sputtered a-SiCxNy thin films with various nitrogen content (0–40 at.%) and C/Si close to one deposited on silicon substrates were evaluated before and after exposure to high temperatures up to 1100 °C in air and vacuum. IR transmission spectroscopy revealed formation of multiple C-N bonds for the films with N content higher than 30 at.%. Improvement of the organization in the carbon phase with the increase of nitrogen content in the a-SiCN films was detected by Raman spectroscopy. Nanoindentation and scratch test point out on the beneficial effect of the nitrogen doping on the tribo-mechanical performance of a-SiCxNy coatings, especially for the annealed coatings. The improved fracture resistance of the SiCN films stems from the formation of triple C≡N bonds for the as deposited films and also by suppression of SiC clusters crystallization by incorporation of nitrogen atoms for annealed films. This together with higher susceptibility to oxidation of a-SiCN films impart them higher scratch and wear resistance in comparison to SiC films before as well as after the thermal exposure. The best tribo-mechanical performance in term of high hardness and sufficient level of ductility were observed for the a-Si0.32C0.32N0.36 film. The enhanced performance is preserved after the thermal exposure in air (up to 1100 °C) and vacuum (up to 900 °C) atmosphere. Annealing in oxidizing atmosphere has a beneficial effect in terms of tribological properties. Harder films with lower nitrogen content suffer from higher brittleness. FIB-SEM identified film-confined cracking as the initial failure event in SiC, while it was through-interface cracking for SiCN at higher loads. This points out on the higher fracture resistance of the SiCN films where higher strains are necessary for crack formation.
Operation
conditions of new generation of advanced ceramic-based coatings and
thin films often include harsh and high temperature oxidizing
atmospheres or highly loaded mechanical contacts either during the
fabrication process or the service life.
Therefore a high hardness ensuring the resistance to scratch initiation
together with a sufficient amount of ductility preventing brittle large
area failure combined with a high temperature oxidation resistance are of the highest importance for these coatings’ structural applications. An intensive effort has been devoted to a research and development of ultra-durable coatings.
However, during the lifetime the elevated temperature exposure can lead
to the chemical and structural changes of these coatings that result in
change of mechanical and tribological properties. Hence the effect of
thermal exposure on the coatings’ chemical and physical properties
stability has to be carefully and reliably explored.
An exceptional rank in terms of structural ceramic belongs to SiC-based materials that are considered as a very promising candidate with an outstanding potential for diverse applications. This stems from strongly bonded three dimensional structure composed of light elements, resulting in high hardness, superior tribological resistance, chemical and mechanical stability even at elevated temperatures as well as high radiation resistance. Moreover, appropriate doping strategies allow tailoring the optical, electrical and mechanical properties. Especially the band gap engineering together with easy applicability of silicon-based technology make SiC an ideal material for electronic devices (MEMS, sensors transistors, etc.) used even in harsh environments as well as for applications in space optics and electronics. The combination of exceptional high temperature mechanical durability with chemical and radiation resistance makes SiC a serious replacement material for zirconium-based alloys in case of cladding for nuclear fuel or a very promising material for the plasma facing components in nuclear fusion reactors.
Despite the vast application potential of SiC its structural applications are limited by its brittleness, which has hindered its use in contrast to other more sturdy transition-metal carbides or nitride based ceramics. It is especially due to the directional covalent bonds between Si and C atoms that do not allow dislocation based deformation. Crystalline silicon carbide fractures transgranularly along the grain boundaries. Therefore, many efforts have been devoted to optimize the grain size shape or sintering additives. There has been proposed several approaches to improve the toughness of ceramic materials both in bulk and thin films based on tailoring their composition and/or microstructure. Toughness, in general, is defined as the ability of the material to absorb deformation energy before fracture and different methodologies have been developed based on (i) ductile phase incorporation, (ii) grain size refinement and reinforcement, (iii) structure grading, (iv) multilayered concept taking advantage of crack deflection, crack tip blunting due to nanoplasticity and ductile interlayer ligament bridging at the interface between layers or (v) phase transition. This can be accomplished either by the careful choice of technological process conditions or by changing the composition and/or by creation of specific structure. The main process parameters of physical vapor deposition (PVD) of thin films includes electrical power, substrate temperature, working gas, deposition pressure, substrate bias, chamber design etc. In the alternative approach the chemical composition and in turn the mechanical properties of the Si-C-M system, where M stands for transition metal, can be modified depending on the C-M bond strength and the ratio of C/M atomic radii. In this case the atoms are attracted by the mixture of covalent, ionic and metal bonds. Depending on the deposition conditions and actual composition various nanocomposite structures can be also formed. A typical example is nc-TiCx/a-SiC, where TiCx nanograins are embedded in the amorphous a-SiC matrix. In general the Si often leads to reduction of grain size in nc-MCx/a-C system. However the XRD amorphous structures were also reported for Cr-Si-C, Ta-Si-C, Mo-Si-C and W-Si-C systems. Another promising nanostructure based on Si-C system may be represented by the Ti3SiC2 phase or Ti4SiC3 MAX phases. These nanolayered hexagonal ceramics, following the general formula Mn+1AXn (where n = 1,2,3, M is an early transition metal, A is a group A element and X is carbon or nitrogen), combines the merits of metals and ceramic such as high hardness, good machinability together with enhanced ductility.
The possible solution to overcome the high fragility of silicon carbide may be an introduction of nitrogen atoms into its amorphous structure. Especially the amorphous structure is important since in terms of enhanced toughness it is preferable over the crystalline one. Besides, introduction of nitrogen into the Si-C system has been shown to be very promising for achievement of very high thermal stability above 1000 °C, where the transition metal nitrides suffer from extensive oxidation. In fact the a-SixCzNy coatings were reported to possess different properties in comparison to the coatings based on mixture of Si3N4 and SiC and exhibit exceptional thermal stability even above 1500 °C that is considered as an ultimate temperature for Si3N4. The potential of SiCN films has been explored extensively in last few decades and it was reported that the best mechanical properties exhibit films with composition along the SiC–Si3N4 line in the Si–C–N compositional triangle and that the highest thermal stability may be ascribed to the amorphous structure and Si-C bonds. Besides, nitrogen doping has been already reported as a very convenient way to modify band gap of SiCxNy (N-doped SiC) coatings. In case of SiCN thin films and coatings, different approaches based on various types of physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques have been employed using different source materials (targets) and gasses (CH4, C2H2 or SiH4). Since these techniques may be operated under thermodynamically unstable conditions, it is possible to prepare materials with unique structures and compositions far from the thermodynamical equilibrium that are unattainable by standard bulk technologies. Both PVD and CVD techniques in case of SiCxNy thin films and coatings as well as polymer to ceramic transformation-based approaches for SiCxNy bulk have succeeded in reaching exceptional thermal stability. However high values of hardness approaching 20 GPa have been reported mostly for various PVD and CVD methods. This clearly reflects the complexity in tailoring mechanical and chemical resistance and points out on the effect of conditions and peculiarities of the fabrication process on structure and composition of the SiCxNy material.
Regardless of whether the primary application is based on optical, electrical or mechanical properties, also the protective ability, mechanical stability and durability of SiC or SiCN bulk/coatings are equally important. It should be noted that in case of thin films and coatings the strong adhesion to the underlying substrate is also of the highest importance as evidenced above. There have been introduced tens of various tests for evaluation of adhesion/cohesion strength, but in case of layered systems the scratch test has been established as the most reliable and recommended one. This is especially due to its simple principle, high reproducibility and ability to mimic the service load conditions. Despite the vast application potential and already proved applicability of SiC and/or SiCxNy there is a lack of systematic data on the adhesion strength of these coatings and only few studies can be cited. This is further underlined by the fact that the adhesion failure is often the primary failure mechanism of coatings. The wear behavior of amorphous Si/C/N:H synthesized by RF plasma enhanced chemical vapor deposition (RF-PECVD) was studied in sliding tests under oscillating motion and brittle nature combined with rather week adhesion were reported. The adhesion and failure mechanism of RF magnetron sputtered SiC thin films on steel and nanocrystalline Si-C-N films on Si were also reported and compared to a-CNx in latter case.
In previous studies we deposited a-SiCxNy films with various N content ranging from 0 to 40 at.% and thoroughly analyzed their structural and compositional stability after air and vacuum annealing as well as the high temperature mechanical properties. This paper expands the scope of earlier works by focusing on the adhesion and tribological performance of these amorphous SiCxNy coatings before and after high temperature annealing at 700, 900 and 1100 °C both in air and vacuum and relate them to structure, composition and mechanical properties. The standard ramped scratch test and repetitive scratch test (multi-pass wear test) were employed to explore both the effect of N content and the annealing temperature. In addition, the methodology of nano/micro scratch test evaluation is reported and the extraordinary performance of acoustic emission-based approach is demonstrated.
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An exceptional rank in terms of structural ceramic belongs to SiC-based materials that are considered as a very promising candidate with an outstanding potential for diverse applications. This stems from strongly bonded three dimensional structure composed of light elements, resulting in high hardness, superior tribological resistance, chemical and mechanical stability even at elevated temperatures as well as high radiation resistance. Moreover, appropriate doping strategies allow tailoring the optical, electrical and mechanical properties. Especially the band gap engineering together with easy applicability of silicon-based technology make SiC an ideal material for electronic devices (MEMS, sensors transistors, etc.) used even in harsh environments as well as for applications in space optics and electronics. The combination of exceptional high temperature mechanical durability with chemical and radiation resistance makes SiC a serious replacement material for zirconium-based alloys in case of cladding for nuclear fuel or a very promising material for the plasma facing components in nuclear fusion reactors.
Despite the vast application potential of SiC its structural applications are limited by its brittleness, which has hindered its use in contrast to other more sturdy transition-metal carbides or nitride based ceramics. It is especially due to the directional covalent bonds between Si and C atoms that do not allow dislocation based deformation. Crystalline silicon carbide fractures transgranularly along the grain boundaries. Therefore, many efforts have been devoted to optimize the grain size shape or sintering additives. There has been proposed several approaches to improve the toughness of ceramic materials both in bulk and thin films based on tailoring their composition and/or microstructure. Toughness, in general, is defined as the ability of the material to absorb deformation energy before fracture and different methodologies have been developed based on (i) ductile phase incorporation, (ii) grain size refinement and reinforcement, (iii) structure grading, (iv) multilayered concept taking advantage of crack deflection, crack tip blunting due to nanoplasticity and ductile interlayer ligament bridging at the interface between layers or (v) phase transition. This can be accomplished either by the careful choice of technological process conditions or by changing the composition and/or by creation of specific structure. The main process parameters of physical vapor deposition (PVD) of thin films includes electrical power, substrate temperature, working gas, deposition pressure, substrate bias, chamber design etc. In the alternative approach the chemical composition and in turn the mechanical properties of the Si-C-M system, where M stands for transition metal, can be modified depending on the C-M bond strength and the ratio of C/M atomic radii. In this case the atoms are attracted by the mixture of covalent, ionic and metal bonds. Depending on the deposition conditions and actual composition various nanocomposite structures can be also formed. A typical example is nc-TiCx/a-SiC, where TiCx nanograins are embedded in the amorphous a-SiC matrix. In general the Si often leads to reduction of grain size in nc-MCx/a-C system. However the XRD amorphous structures were also reported for Cr-Si-C, Ta-Si-C, Mo-Si-C and W-Si-C systems. Another promising nanostructure based on Si-C system may be represented by the Ti3SiC2 phase or Ti4SiC3 MAX phases. These nanolayered hexagonal ceramics, following the general formula Mn+1AXn (where n = 1,2,3, M is an early transition metal, A is a group A element and X is carbon or nitrogen), combines the merits of metals and ceramic such as high hardness, good machinability together with enhanced ductility.
The possible solution to overcome the high fragility of silicon carbide may be an introduction of nitrogen atoms into its amorphous structure. Especially the amorphous structure is important since in terms of enhanced toughness it is preferable over the crystalline one. Besides, introduction of nitrogen into the Si-C system has been shown to be very promising for achievement of very high thermal stability above 1000 °C, where the transition metal nitrides suffer from extensive oxidation. In fact the a-SixCzNy coatings were reported to possess different properties in comparison to the coatings based on mixture of Si3N4 and SiC and exhibit exceptional thermal stability even above 1500 °C that is considered as an ultimate temperature for Si3N4. The potential of SiCN films has been explored extensively in last few decades and it was reported that the best mechanical properties exhibit films with composition along the SiC–Si3N4 line in the Si–C–N compositional triangle and that the highest thermal stability may be ascribed to the amorphous structure and Si-C bonds. Besides, nitrogen doping has been already reported as a very convenient way to modify band gap of SiCxNy (N-doped SiC) coatings. In case of SiCN thin films and coatings, different approaches based on various types of physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques have been employed using different source materials (targets) and gasses (CH4, C2H2 or SiH4). Since these techniques may be operated under thermodynamically unstable conditions, it is possible to prepare materials with unique structures and compositions far from the thermodynamical equilibrium that are unattainable by standard bulk technologies. Both PVD and CVD techniques in case of SiCxNy thin films and coatings as well as polymer to ceramic transformation-based approaches for SiCxNy bulk have succeeded in reaching exceptional thermal stability. However high values of hardness approaching 20 GPa have been reported mostly for various PVD and CVD methods. This clearly reflects the complexity in tailoring mechanical and chemical resistance and points out on the effect of conditions and peculiarities of the fabrication process on structure and composition of the SiCxNy material.
Regardless of whether the primary application is based on optical, electrical or mechanical properties, also the protective ability, mechanical stability and durability of SiC or SiCN bulk/coatings are equally important. It should be noted that in case of thin films and coatings the strong adhesion to the underlying substrate is also of the highest importance as evidenced above. There have been introduced tens of various tests for evaluation of adhesion/cohesion strength, but in case of layered systems the scratch test has been established as the most reliable and recommended one. This is especially due to its simple principle, high reproducibility and ability to mimic the service load conditions. Despite the vast application potential and already proved applicability of SiC and/or SiCxNy there is a lack of systematic data on the adhesion strength of these coatings and only few studies can be cited. This is further underlined by the fact that the adhesion failure is often the primary failure mechanism of coatings. The wear behavior of amorphous Si/C/N:H synthesized by RF plasma enhanced chemical vapor deposition (RF-PECVD) was studied in sliding tests under oscillating motion and brittle nature combined with rather week adhesion were reported. The adhesion and failure mechanism of RF magnetron sputtered SiC thin films on steel and nanocrystalline Si-C-N films on Si were also reported and compared to a-CNx in latter case.
In previous studies we deposited a-SiCxNy films with various N content ranging from 0 to 40 at.% and thoroughly analyzed their structural and compositional stability after air and vacuum annealing as well as the high temperature mechanical properties. This paper expands the scope of earlier works by focusing on the adhesion and tribological performance of these amorphous SiCxNy coatings before and after high temperature annealing at 700, 900 and 1100 °C both in air and vacuum and relate them to structure, composition and mechanical properties. The standard ramped scratch test and repetitive scratch test (multi-pass wear test) were employed to explore both the effect of N content and the annealing temperature. In addition, the methodology of nano/micro scratch test evaluation is reported and the extraordinary performance of acoustic emission-based approach is demonstrated.
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