Periodic Report Summary 1 - ISATRIACAM (In Situ Analytical Tribology for Investigating Advanced Carbon-Based Materials)
Diamond and diamond-like carbon coatings are widely used in a broad range of technologically important applications thanks to their impressive properties (mechanical, electrical, optical, and thermal, for example) [1]. The extensive study of these materials has allowed, to a significant degree, the correlation of their properties (i.e. structure and composition) with their functional behaviour (e.g. mechanical properties), although further work is needed. Conversely, in the case of novel diamond-like carbon coatings (such as silicon oxide-doped diamond-like carbon - SiOx-DLC) which are tailored to meet the ever-increasing performance and durability requirements of advanced industrial applications, a fundamental understanding of the materials property-functional behaviour relationships (i.e. the interrelationship between composition, structure, tribological performance and durability) has not been established yet.
Silicon oxide-doped diamond-like carbon (SiOx-DLC) (sometimes referred to as diamond-like nanocomposite, DLN) is a family of materials, which are promising for several engineering applications as they have potential to exhibit extremely good tribological properties (low friction and wear) across a range of conditions and environments. SiOx-DLCs are fully amorphous coatings supposedly consisting of two interpenetrating, interbonded networks, one being an hydrogenated amorphous carbon (a-C:H) and the other a silica glass (SiOx) network [2]. These films’ potential for applications derives from the combination of mechanical properties comparable to those of a-C:H films, but with additional benefits including superior thermal stability and oxidation resistance, low residual stresses, and good tribological behaviour even under harsh conditions (including elevated temperatures, high contact stresses, ultrahigh vacuum, and humid/oxidizing environments). However, the lack of fundamental understanding of the functional behaviour of these materials has hampered the possibility of revealing critical materials properties whose knowledge is crucial for tailoring the deposition process with the aim of depositing coatings with the required performance.
The proposed research project lies in seeking, for the first time, a fundamental understanding of the dependence of the functional behaviour (namely thermal stability and tribological properties) on materials properties (i.e. structure and composition) for SiOx-DLC thin films. In fact, studies where tribological mechanisms are deeply and comprehensively understood through fundamental links to the structure and properties of the materials involved are not common, because a large array of tools and an interdisciplinary approach are both required. An innovative outcome of this approach is to provide a starting point for rationally designing modified carbon-based materials with substantially improved properties.
The main objectives of the proposed project are:
a) Determine the structure and composition of both the bulk and surface of SiOx-DLC films, and correlate them with the material’s physico-chemical and mechanical properties.
Even though the interpenetration of the two networks constituting SiOx-DLC, i.e. hydrogenated amorphous carbon and silica glass, has been postulated to be complete [3, 4], no conclusive model for the structure(s) of SiOx-DLC has ever been proposed. The determination of the bulk and surface structural properties, such as the extent to which the silica and hydrogenated amorphous carbon phases are interbonded or segregated, the distribution of the carbon hybridization states, the silicon oxidation states, etc., are of paramount importance for correlating material properties and functional behaviour.
b) Assess the thermal stability of SiOx-DLC and develop a fundamental understanding of the influence of the environment on the thermal degradation pathway and kinetics of SiOx-DLC.
Even though it is well established that SiOx-DLC possesses superior thermal stability and oxidation resistance than conventional a-C:H films, the scientific basis for this behaviour of SiOx-DLC is not established. This motivates experimental efforts to determine the onset temperature for the degradation of SiOx-DLC, the decomposition pathway, and the influence of the environment on the degradation kinetics.
c) Establish the scientific basis for the environmental influences on the tribological performance of SiOx-DLC.
While the tribological properties of SiOx-DLC have been extensively investigated in different environments, to date only a few studies have focused on the mechanisms relating environment and tribological performance. Notably, these studies were performed ex situ, i.e. outside the tribometer and after the friction experiment [5]. Although this enables the application of several complementary modern surface-analytical techniques [6], the specimens usually must be characterized in an environment that is very different from that found within the tribosystem. The inevitable air exposure during the transfer of the sample from the tribometer to the analytical instrument, even when performed as rapidly as possible, can render the surface unrepresentative of its state during sliding. Thus, in situ tribological methods need to be employed to gain an understanding of the fundamental mechanisms underlying the effect of the environment on the tribological performance of SiOx-DLC.
The main scientific achievements obtained during the outgoing phase of the proposed project (duration of the phase: 18 months) are listed in the following.
• The structure and composition of the bulk and the surface of SiOx-DLC was determined using multiple surface-analytical and bulk characterization techniques. This required the combination of standard, laboratory-based methods and advanced techniques (i.e. synchrotron-based spectroscopy and solid-state nuclear magnetic resonance spectroscopy). The analytical results allowed the development of a structural model for SiOx-DLC, thus bridging a gap in the published literature and providing structural information (such as the extent to which the silica and hydrogenated amorphous carbon phases are interbonded or segregated, the distribution of the carbon hybridization states, the silicon oxidation states) of paramount importance for correlating material properties and functional behaviour (paper in preparation);
• The investigation of the surface structure of amorphous carbon-based films led to the development of a novel methodology for determining the carbon hybridization state in the near-surface region of these materials on the basis of X-ray photoelectron spectroscopy (XPS) data (paper published in Applied Physics Letters);
• The range of energy barriers associated with specific processes involved in the thermal decomposition of amorphous carbon-based films was quantified. Via careful characterization of the surface structure and chemistry of amorphous carbon-based films, it was identified clustering and ordering of the sp2 carbon phase, scission of carbon-hydrogen bonds with formation of sp2 carbon, and direct transformation of sp3- to sp2-hybridized carbon as the key mechanisms and quantified the activation energy ranges involved (paper published in Applied Physics Letters);
• A new methodology for the analysis of multilayer structures by NEXAFS spectroscopy was performed (paper published in Analytical Chemistry). The method allows the contribution of adventitious carbon contamination to be removed from the as-acquired spectra of carbon-based films previously exposed to air to give the intrinsic photo-absorption NEXAFS spectra of the material under investigation;
• Through careful investigation of the surface chemical reactions and structural transformation occurring on SiOx-DLC at elevated temperatures and under different environmental conditions, the thermally-activated physical processes occurring in this material were identified while quantifying their activation energy barriers. Furthermore, the major role of silicon and oxygen in SiOx-DLC in enhancing the stability of this material at elevated temperatures under aerobic conditions compared to hydrogenated amorphous carbon films was pinpointed (4 papers in preparation);
• Through active interactions with a start-up company (NCD Technologies, Madison, WI, US), SiOx-DLC films with tailored composition and structure were developed with the aim of depositing ultra-thin films suitable for the systematic investigation of the interrelationship between composition, structure, tribological performance and durability;
• Through the use of an advanced synchrotron-based spectroscopic technique (i.e. magnetically-guided imaging NEXAFS spectroscopy at the National Synchrotron Light Source, Brookhaven National Laboratory, NY, U.S.) the role of the gaseous environment on the structural transformations occurring on SiOx-DLC upon sliding was identified. The work was carried out collaboratively with Ecole Centrale de Lyon (paper in preparation);
• An in situ tribological test apparatus was designed to be installed inside the E-XPS chamber. The purpose of this novel tool is to establish the scientific basis for the environmental influence on the tribological performance of solid lubricants. While the tribological test apparatus provides information about the frictional performance, E-XPS analyses are carried out in situ with the aim of investigating surface chemical reactions and structural changes affecting the observed tribological response.
In summary, the proposed research work, which has resulted in the publication of 2 peer-reviewed journal articles so far with 6 more in preparation, led to a better understanding of the surface phenomena occurring on carbon-based materials in response to temperature or other energetic inputs. This work is currently continuing at the Laboratoire de Tribologie Dynamique des Systèmes (Ecole Centrale de Lyon, Lyon, France), where Dr. F. Mangolini’s work focuses on the surface-analytical investigation of the structural and chemical changes occurring on DLC and SiOx-DLC upon sliding using synchrotron-based X-ray techniques with unparalleled spatial resolution (down to 30 nm. Measurements carried out at SOLEIL Synchrotron Facility, Paris, France). The exceptional lateral resolution with which XPS and NEXAFS analysis can be performed at SOLEIL provides the possibility, for the first time, of characterizing (with extreme surface sensitivity) the chemistry and structure of the materials active in the mechanically-stressed region at length scale comparable to contact asperities.
References
1. Donnet, C. and A. Erdemir, eds. Tribology of Diamond-Like Carbon Films. 2008, Springer: New York. 664.
2. Scharf, T.W. et al., Journal of Applied Physics, 2007. 101(6): p. 063521-11.
3. Dorfman, V.F. Thin Solid Films, 1992. 212(1-2): p. 267-273.
4. Neerinck, D., et al., Diamond and Related Materials, 1998. 7(2-5): p. 468-471.
5. Donnet, C., Problem-Solving Methods in Tribology with Surface-Specific Techniques, in Handbook of Surface and Interface Analysis: Methods and Problem-Solving, J.C. Rivière and S. Myhra, Editors. 1998, Marcel Dekker Inc.: New York. p. 968.
6. Gellman, A.J. and N.D. Spencer, Journal of Engineering Tribology, 2002. 216(6): p. 443-461.
Silicon oxide-doped diamond-like carbon (SiOx-DLC) (sometimes referred to as diamond-like nanocomposite, DLN) is a family of materials, which are promising for several engineering applications as they have potential to exhibit extremely good tribological properties (low friction and wear) across a range of conditions and environments. SiOx-DLCs are fully amorphous coatings supposedly consisting of two interpenetrating, interbonded networks, one being an hydrogenated amorphous carbon (a-C:H) and the other a silica glass (SiOx) network [2]. These films’ potential for applications derives from the combination of mechanical properties comparable to those of a-C:H films, but with additional benefits including superior thermal stability and oxidation resistance, low residual stresses, and good tribological behaviour even under harsh conditions (including elevated temperatures, high contact stresses, ultrahigh vacuum, and humid/oxidizing environments). However, the lack of fundamental understanding of the functional behaviour of these materials has hampered the possibility of revealing critical materials properties whose knowledge is crucial for tailoring the deposition process with the aim of depositing coatings with the required performance.
The proposed research project lies in seeking, for the first time, a fundamental understanding of the dependence of the functional behaviour (namely thermal stability and tribological properties) on materials properties (i.e. structure and composition) for SiOx-DLC thin films. In fact, studies where tribological mechanisms are deeply and comprehensively understood through fundamental links to the structure and properties of the materials involved are not common, because a large array of tools and an interdisciplinary approach are both required. An innovative outcome of this approach is to provide a starting point for rationally designing modified carbon-based materials with substantially improved properties.
The main objectives of the proposed project are:
a) Determine the structure and composition of both the bulk and surface of SiOx-DLC films, and correlate them with the material’s physico-chemical and mechanical properties.
Even though the interpenetration of the two networks constituting SiOx-DLC, i.e. hydrogenated amorphous carbon and silica glass, has been postulated to be complete [3, 4], no conclusive model for the structure(s) of SiOx-DLC has ever been proposed. The determination of the bulk and surface structural properties, such as the extent to which the silica and hydrogenated amorphous carbon phases are interbonded or segregated, the distribution of the carbon hybridization states, the silicon oxidation states, etc., are of paramount importance for correlating material properties and functional behaviour.
b) Assess the thermal stability of SiOx-DLC and develop a fundamental understanding of the influence of the environment on the thermal degradation pathway and kinetics of SiOx-DLC.
Even though it is well established that SiOx-DLC possesses superior thermal stability and oxidation resistance than conventional a-C:H films, the scientific basis for this behaviour of SiOx-DLC is not established. This motivates experimental efforts to determine the onset temperature for the degradation of SiOx-DLC, the decomposition pathway, and the influence of the environment on the degradation kinetics.
c) Establish the scientific basis for the environmental influences on the tribological performance of SiOx-DLC.
While the tribological properties of SiOx-DLC have been extensively investigated in different environments, to date only a few studies have focused on the mechanisms relating environment and tribological performance. Notably, these studies were performed ex situ, i.e. outside the tribometer and after the friction experiment [5]. Although this enables the application of several complementary modern surface-analytical techniques [6], the specimens usually must be characterized in an environment that is very different from that found within the tribosystem. The inevitable air exposure during the transfer of the sample from the tribometer to the analytical instrument, even when performed as rapidly as possible, can render the surface unrepresentative of its state during sliding. Thus, in situ tribological methods need to be employed to gain an understanding of the fundamental mechanisms underlying the effect of the environment on the tribological performance of SiOx-DLC.
The main scientific achievements obtained during the outgoing phase of the proposed project (duration of the phase: 18 months) are listed in the following.
• The structure and composition of the bulk and the surface of SiOx-DLC was determined using multiple surface-analytical and bulk characterization techniques. This required the combination of standard, laboratory-based methods and advanced techniques (i.e. synchrotron-based spectroscopy and solid-state nuclear magnetic resonance spectroscopy). The analytical results allowed the development of a structural model for SiOx-DLC, thus bridging a gap in the published literature and providing structural information (such as the extent to which the silica and hydrogenated amorphous carbon phases are interbonded or segregated, the distribution of the carbon hybridization states, the silicon oxidation states) of paramount importance for correlating material properties and functional behaviour (paper in preparation);
• The investigation of the surface structure of amorphous carbon-based films led to the development of a novel methodology for determining the carbon hybridization state in the near-surface region of these materials on the basis of X-ray photoelectron spectroscopy (XPS) data (paper published in Applied Physics Letters);
• The range of energy barriers associated with specific processes involved in the thermal decomposition of amorphous carbon-based films was quantified. Via careful characterization of the surface structure and chemistry of amorphous carbon-based films, it was identified clustering and ordering of the sp2 carbon phase, scission of carbon-hydrogen bonds with formation of sp2 carbon, and direct transformation of sp3- to sp2-hybridized carbon as the key mechanisms and quantified the activation energy ranges involved (paper published in Applied Physics Letters);
• A new methodology for the analysis of multilayer structures by NEXAFS spectroscopy was performed (paper published in Analytical Chemistry). The method allows the contribution of adventitious carbon contamination to be removed from the as-acquired spectra of carbon-based films previously exposed to air to give the intrinsic photo-absorption NEXAFS spectra of the material under investigation;
• Through careful investigation of the surface chemical reactions and structural transformation occurring on SiOx-DLC at elevated temperatures and under different environmental conditions, the thermally-activated physical processes occurring in this material were identified while quantifying their activation energy barriers. Furthermore, the major role of silicon and oxygen in SiOx-DLC in enhancing the stability of this material at elevated temperatures under aerobic conditions compared to hydrogenated amorphous carbon films was pinpointed (4 papers in preparation);
• Through active interactions with a start-up company (NCD Technologies, Madison, WI, US), SiOx-DLC films with tailored composition and structure were developed with the aim of depositing ultra-thin films suitable for the systematic investigation of the interrelationship between composition, structure, tribological performance and durability;
• Through the use of an advanced synchrotron-based spectroscopic technique (i.e. magnetically-guided imaging NEXAFS spectroscopy at the National Synchrotron Light Source, Brookhaven National Laboratory, NY, U.S.) the role of the gaseous environment on the structural transformations occurring on SiOx-DLC upon sliding was identified. The work was carried out collaboratively with Ecole Centrale de Lyon (paper in preparation);
• An in situ tribological test apparatus was designed to be installed inside the E-XPS chamber. The purpose of this novel tool is to establish the scientific basis for the environmental influence on the tribological performance of solid lubricants. While the tribological test apparatus provides information about the frictional performance, E-XPS analyses are carried out in situ with the aim of investigating surface chemical reactions and structural changes affecting the observed tribological response.
In summary, the proposed research work, which has resulted in the publication of 2 peer-reviewed journal articles so far with 6 more in preparation, led to a better understanding of the surface phenomena occurring on carbon-based materials in response to temperature or other energetic inputs. This work is currently continuing at the Laboratoire de Tribologie Dynamique des Systèmes (Ecole Centrale de Lyon, Lyon, France), where Dr. F. Mangolini’s work focuses on the surface-analytical investigation of the structural and chemical changes occurring on DLC and SiOx-DLC upon sliding using synchrotron-based X-ray techniques with unparalleled spatial resolution (down to 30 nm. Measurements carried out at SOLEIL Synchrotron Facility, Paris, France). The exceptional lateral resolution with which XPS and NEXAFS analysis can be performed at SOLEIL provides the possibility, for the first time, of characterizing (with extreme surface sensitivity) the chemistry and structure of the materials active in the mechanically-stressed region at length scale comparable to contact asperities.
References
1. Donnet, C. and A. Erdemir, eds. Tribology of Diamond-Like Carbon Films. 2008, Springer: New York. 664.
2. Scharf, T.W. et al., Journal of Applied Physics, 2007. 101(6): p. 063521-11.
3. Dorfman, V.F. Thin Solid Films, 1992. 212(1-2): p. 267-273.
4. Neerinck, D., et al., Diamond and Related Materials, 1998. 7(2-5): p. 468-471.
5. Donnet, C., Problem-Solving Methods in Tribology with Surface-Specific Techniques, in Handbook of Surface and Interface Analysis: Methods and Problem-Solving, J.C. Rivière and S. Myhra, Editors. 1998, Marcel Dekker Inc.: New York. p. 968.
6. Gellman, A.J. and N.D. Spencer, Journal of Engineering Tribology, 2002. 216(6): p. 443-461.