ICMCTF2015 Session B1-2: PVD Coatings and Technologies

Ti-Al-O-N coatings were synthesized by cathodic arc and high power pulsed magnetron sputtering. The chemical composition of the coatings was determined by means of elastic recoil detection analysis and energy dispersive X-ray spectroscopy. The effect of oxygen incorporation on the stress-free lattice parameters and Young’s moduli of Ti-Al-O-N coatings was investigated by X-ray diffraction and nanoindentation, respectively. As nitrogen is substituted by oxygen, implications for the charge balance may be expected. A reduction in equilibrium volume with increasing O concentration is identified by X-ray diffraction and density functional theory calculations of Ti-Al-O-N supercells reveal the concomitant formation of metal vacancies. Hence, the oxygen incorporation-induced formation of metal vacancies enables charge balancing. Furthermore, nanoindentation experiments reveal a decrease in elastic modulus with increasing O concentration. Based on ab initio data, two causes can be identified for this: First, the metal vacancy-induced reduction in elasticity; and second, the formation of, compared to the corresponding metal nitride bonds, relatively weak Ti-O and Al-O bonds.

Forming tools are subjected to a high level of cyclic loads. This results in an increased rate of abrasive and adhesive wear as well as fatigue of the tool material. Therefore, application of wear and fatigue resistant coatings may decrease the deterioration rate of the tool resulting in a longer tool life. This along with the less frequent maintenance of the tool/die gives rise to the productivity of the whole system. Commercially available nitride-based coatings have shown great potential for such purposes. However, in many cases these coatings were originally aimed for improvement of cutting tools functionalities where the failure mechanisms may differ significantly compared to those in forming tool applications. Development of more fatigue and wear-resistant coatings devoted to forming tool applications is further motivated by the increasing utilization of AHSS metal sheets used in e.g. automotive industry. In this respect, the role of alloying elements in improvement of wear and fatigue resistance of the binary, ternary and quaternary nitride-based coatings are of great potential for more intensive investigations.

Boron carbonitride (BCN) is a promising candidate for wear protection applications, because of its mechanical and tribological properties, such as hardness, coefficient of friction, wear resistance and temperature stability. In this work, we investigate the influence of the composition of such BCN coatings on the mechanical and tribological properties of wear resistant films.

In a previous study we focused on BCN coatings with a B:C ratio of 4:1 and variable nitrogen contents grown by sputtering from a B₄C target in pulsed DC or High Power Impulse Magnetron Sputtering (HiPIMS) mode. The film’s structure in this area of the ternary phase diagram was amorphous and showed high hardness, but coefficients of friction in dry testing conditions were comparably high.

In this study, we aim to improve the film’s tribological behavior by modifying the film’s carbon content by reactive sputtering with acetylene or pulsed DC co-sputtering from a graphite target. Sputtering with acetylene is advantageous for industrial applications due to a high deposition rate, however, reactive sputtering with acetylene leads to an incorporation of hydrogen into the coating structure which is detrimental to the temperature stability of the films. Therefore, we compare the tribological properties and temperature stability of films deposited by reactive sputtering with acetylene and hydrogen-free BCN films. Furthermore, we discuss the influence of the high ionization of plasma species found in HiPIMS observed by optical emission spectroscopy on the film composition and resulting mechanical properties and temperature stability.

Finally, we correlate the coating composition determined by X-ray photoelectron spectroscopy with topography and morphology examined by scanning electron microscopy and hardness as well as adhesive properties determined by nano- and Rockwell indentation.

Through Ar/N₂ inlet gas ratio control during magnetron sputtering, Tantalum nitrides (TaN) and Hafnium nitrides (HfN) layers were manipulated with amorphous and crystalline structure .The columnar crystalline structures of TaN and HfN were fabricated under Ar/N₂ ratios of 18/2 and 19/1 sccm/sccm respectively, while the amorphous structure appeared at 12/8 and 17/3. The thickness, deposition rate, as well as crystallinity of TaN and HfN decreased with increasing N₂ flow. The X-ray diffraction patterns of TaN under the ratio of 18/2 showed strong peaks of TaN phase with a preferred orientation (200). The TaN crystalline feature diminished at a higher N₂ flow. For HfN, the X-ray diffraction patterns showed the phase transition from 18/2 to 17/3 of Ar/N₂ ratio. The surface condition of HfN was smooth relatively and the surface roughness (Ra) slightly increased with N₂ flow. To enhance the mechanical properties of TaN and HfN films, crystalline and amorphous layers with various bilayer thickness and ratios were alternately stacked into nano-multilayers, i.e. c-TaN/a-TaN and c-HfN/a-HfN. The multilayer coating has slightly lower hardness and Young’s modulus, but shows a better adhesion and lower friction coefficient than single layer coating. Characterization of multilayer amorphous/crystalline coatings was investigated and discussed.

Al doped ZnO(AZO) thin film has been attracting as one of promising candidate film replacing ITO film for transparent conductive oxide film of next generation flexible digital electronics devices. The relatively low resistivity and transparency of AZO film deposited at low temperature are still hurdles to overcome for replacing ITO film even though its unique advantages in low cost and high toughness over those of ITO film. It is well known that resistivity is closely associated with carrier concentration and mobility which are controlled by stoichiometry, binding energy of atoms and lattice defects of film including oxygen vacancy as well as Al replacement in Zn atom sites. The control of those atomistic structures and lattice defects are affected by surface energy accumulated with atoms and molecules deposited at top surface layer during film nucleation and growth depending on process parameters during deposition process. The surface energy is mostly comprised of kinetic energy of neutrals, electronic energy of activated neutral molecule and atoms and ions as well as flux density.

We have investigated effects of those atomic and molecular level energy analysis on structure formation and related electrical property changes by in-situ diagnostics during sputtering process under dual confined high density magnetic field. The kinetic energy and flux of sputtered atoms are controlled by independent variation of power density for direct sputtering and in-direct sputtering targets on polymer substrate at low temperature. Optical emission spectroscopy and radical diagnostics as well as Langmuir probe analysis have been performed to measure plasma parameters. Carrier concentration and mobility have been analyzed depending on microstructure changes including binding energy of atoms and Al replacement of Zn site etc.. The resistivity is significantly reduced with atomic level nano process control and can be reached less than 6E-4 at low temperature. This paper discusses on fundamental mechanism of film nucleation and growth with top surface energy accumulation with atomic and molecular energy diagnostics for AZO film synthesis by dual confined high density magnetic field, and illustrates control of resistivity associated with control of carrier concentration and mobility. Also, the transmittance of the films is controlled with optimization of high plasma density.

Biological materials are often functionally optimized through million years of evolution. Abalone nacre possesses exceptional mechanical properties to prevent predatory. Consisting of 95 wt.% CaCO₃ with only 5 wt.% organics contents, nacre has formed a unique “brick-and-mortar” microstructure which significantly enhanced its toughness. Arthropod exoskeleton has multilayer twisted plywood structure with graded mechanical properties, consisting of hard external exocuticle and tough endocuticle, which give better impact resistance. Sponge spicules are composed mainly of SiO₂ and small amount of proteins organized into a concentric structure. The organic/inorganic interfaces prevent crack propagation and significantly enhance fracture toughness. Inspired from the organic/inorganic multilayer composites in nature, a novel PVD system combining reactive RF sputtering and pulsed laser deposition is designed and utilized. The ceramic constituents are coated by RF sputtering and polymer phases are synthesized by ultraviolet pulsed laser deposition. Three examples will be illustrated in this talk. Firstly, ceramic (ZrN, ZrO₂) and polymer (PMMA, polyimide) multilayer films were successfully fabricated to mimic the abalone nacre structure. By changing the thickness ratio and interfacial roughness of organic and inorganic layers, better fracture toughness of thin films was obtained. Secondly, a multifunctional composite composed of hard ZrO₂ outer layers and tough TiO₂/polyimide inner multilayers was designed to mimic the arthropod exoskeletons, resulting in a hybrid coating with both good abrasion resistance and high impact resistance. Finally, SiO₂/PDMS multilayer thin films inspired from sponge spicules, which has high transparency and crack resistance, were synthesized and can be applied as flexible optical device. The energy-based micro-indentation fracture toughness test was performed to evaluate the fracture toughness of thin films, while the impact resistance of protective coatings was measured by cyclic dynamic loading on specimens. Through these studies, toughening mechanisms were elucidated and optimal bio-inspired designs for novel multifunctional coatings were established.

Cr-Cu-N nanocomposite coatings of varying copper content were produced using an unbalanced magnetron sputtering system. Pulsed DC (PDC) and fixed DC (FDC) bias voltages were applied to the substrates, in order to investigate the effects of a direct pulsed bias on the coating-substrate adhesion, hardness, elastic modulus and microstructure. Coating adhesion strength was measured by scratch testing. Coating surface and cross-sectional morphology were observed by high-resolution field-emission gun scanning electron microscopy (FEG-SEM). The hardness and elastic modulus were measured using nanoindentation. The chemical and phase composition were investigated using energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD).