Development of Isolated Carbon Particles Embedded Diamond-like Carbon Coating - A New Coating Architecture for Improved Mechanical Properties and Tribological Performance

碳粒子嵌入類金剛石塗層的開發 - 一種能夠提高力學性質和摩擦學性能的新型塗層結構

Student thesis: Doctoral Thesis

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Award date11 Dec 2017

Abstract

The research aims to improve the mechanical properties, toughness and wear resistance of diamond-like carbon (DLC) coatings by means of embedding isolated carbon nanoparticles in amorphous DLC film. DLC coatings are popular due to high hardness and good wear resistance. They are being used in various industrial sectors like automotive, cutting tools, biomedical devices, optical devices, watch making etc due to their attractive properties. These coatings had a global share of $0.8bn in 2005 which increased to $1.7bn in 2015. The compound annual growth rate is gradually increasing by every year as several new markets are emerging with latest DLC applications like thin-film sensors, audio speakers, and the use of DLC as a dielectric layer in electrical components. Along with high hardness and good wear resistance, the hard DLC coatings possess low toughness and high residual stress. These factors limit their use for widespread applications. The development of DLC is 35 years old, thus hundreds of studies have been reported to improve DLC performance or to reduce their limitations. DLC doped with foreign elements is an important area in the last 20 years to improve DLC properties. However, the general outcomes suggest that doped DLC may improve toughness, friction behavior or residual stresses, but at the same time it most probably will reduce hardness and increase the wear loss. Therefore, a method to improve all of the major properties in a single coating design remains inspiring.

In this research, the existing methods to improve DLC properties and performance has been critically reviewed. The experimental work was started with in-house deposition and comparison of DLC properties against metal (chromium) and metal-ceramic (chromium diboride) coatings. The comparison based on Oliver and Pharr method for fracture toughness measurement suggested that DLC was tougher than metal and metal-ceramic coatings due to its unique combination of hardness and Young’s modulus. Afterward, DLC coatings were studied in detail by considering the effect of bias voltage and the coating thickness on hardness, toughness and wear performance. A DLC with high hardness but possess low toughness was selected as the baseline. Initially, the improvement in DLC coating had examined based on the recent trends such as DLC/CNT (carbon nanotube) bi-layer structure, chromium doping in DLC and furnace heat treatment. In furnace heat treatment, a new 2-step heat treatment method was introduced which performed better than conventional 1-step heat treatment method. However, in general, properties like toughness or friction coefficients were improved but hardness and wear resistance dropped.

To overcome the existing conflict in improvement and compromise at various DLC properties, this research has proposed a new DLC coating architecture - isolated carbon particles embedded amorphous DLC coating matrix. In the new architecture, the carbon particles will form new bonds with the host DLC matrix and it is anticipated that overall coating hardness will increase due to new sp3 bonds and dense structure. Similarly, the particles are expected to absorb fracture energy and the crack propagation will gradually reduce due to crack deflection, crack bifurcation or crack tip blunting, thus the fracture toughness would be increased. Moreover, the carbon particles having lower shear strength than DLC matrix are expected to improve the sliding wear resistance.

The first challenge of the research is to produce isolated carbon particles with controlled size, and good distribution. The literature suggests that the carbon particles agglomerate when produced with plasma quenching in physical vapor deposition methods. Hence the scientific community, particularly those group who have a good contribution in the synthesis of carbon entities such as Goree, Amaratunga, Chhowalla, Musil, Boucha etc, remained inspired to produce isolated carbon particles by in-situ methods. Based on the literature review and fundamental understanding, a general four-phase model has been proposed in this research. Based on this generalized model a detailed parametric study was designed in which 48 specimens were prepared with different quenching conditions. In the preliminary study, the isolated carbon particles were created on top of DLC coating by quenching Ar plasma with He pulses near the end of DLC deposition. The experimental variables were (1) plasma plume and He pulses orientation, (2) He injection duration, (3) amount of He gas and (4) target to substrate distance. The existence of particles along with their crystalline structure was identified by field emission electron microscope (FESEM) and X-ray diffraction (XRD). Raman spectroscopy was performed to analyze the change in atomic structure. Similarly, the changes in hardness, Young’s modulus, toughness, friction coefficient, wear volume and wear rate were investigated. The plasma quenching outcomes could be classified into three categories,
(1) Agglomerated structures of up to ~2μm size made of mainly ~150±50nm size carbon particles.
(2) A mixture of isolated carbon particles of sizes in the range between ~75nm to ~800nm.
(3) Isolated carbon particles with similar size and spherical shape. The particles were found uniformly distributed all over the surface of 50mm diameter specimen. Two different particles sizes i.e., 300±50nm and 110±30nm, were achieved from this.

The smallest particle size obtained through this parametric study was 110±30nm. The average isolation distance was less than 1μm. Based on the knowledge gained from the first phase work, another set of the parametric study was designed which aimed to further reduce the particle size. In this experimental design, the plasma was quenched with 0.1s He pulses. The variables included (1) number of He pulses, (2) He gas pressure and (3) He flow rate. The outcomes of this parametric study can be described in two aspects:
(1) A mixture of isolated carbon particles having various sizes in the range of ~40nm to ~150nm.
(2) Isolated carbon nanoparticles of 45±10nm size with identical shape and good distribution.

This refined experimental design was capable of creating isolated carbon particles of ~45±10nm size with good repeatability. The particles had good distribution, almost identical shape and an average isolation distance of less than 1μm.

This development has numerous potential applications especially in sensing technology where isolated carbon particles either on the surface or in embedded form are required. Similarly, another potential application could be the antibacterial coatings. The new method could give durable and cost effective solution for above-said applications.

The application of isolated carbon particles is used to design a new DLC coatings architecture and their mechanical properties and tribological performance were examined. In summary, the research enables us to successfully create carbon nanoparticles of a controlled size such as 45±10nm or 110±30nm with an isolation distance of less than 1μm by quenching plasma with He pulses during to DLC deposition. The carbon nanoparticles could be embedded at controlled depths. The baseline DLC coating without particles has high hardness of 32±2GPa, toughness of 1.05±0.3MPa.m1/2 and wear rate of ~7×10-8mm3/Nm. The new coating with embedded carbon particles has ~20% higher hardness than baseline DLC. At the same time, the fracture toughness increased by ~11% and the wear volume and wear rate were reduced by a factor of 3. The in-situ creation of isolated carbon nanoparticles and the simultaneous increase in hardness, toughness, and wear resistance are the major contributions of this research.

In preliminary research, the DLC coatings are deposited on silicon substrate at -120V bias for better scientific understanding of cracking behavior and high accuracy of results obtained from Raman and XRD techniques. However, DLC coatings are being deposited at -80V for typical industrial applications like piston rings and engine valves etc. Thus a case study is designed in which metal substrates are deposited with new 1μm thick DLC coating architecture at -80V bias to examine and verify the newly developed method. Usually the good DLC coatings have wear rate in between 10-7 mm3/Nm to 10-8 mm3/Nm. However, the best coating architecture developed in this research where a single layer of isolated carbon nanoparticles is embedded in amorphous DLC matrix, have a wear rate of ~6×10-9 mm3/Nm when tested at the contact stress of 2.8GPa and 300rpm velocity. At high contact stress of ~3.2GPa, the baseline DLC failed in run-in while all nanoparticles embedded DLC coatings survived at least 2000 cycles at 300rpm velocity. The best results were obtained for the sample which was embedded with least amount of carbon particles (1-layer of carbon nanoparticles) and it survived 6000 cycles. Thus this research suggest that the hardness and toughness can be increased around 10% to 20% and at same time the wear rate can be reduced by a factor of 3 after embedding least amount ( i.e, 1-layer in this work) of isolated carbon nanoparticles in amorphous DLC coatings.

    Research areas

  • DLC, Sputtering, Carbon particls, In-situ, Isolated, Embedded, Mechanical properties, Tribology