Investigations Of Structure And Properties Of Chromium Carbonitride Coatings Produced By Pulsed Closed-Field Unbalanced Magnetron Sputtering

Chromium carbonitride (Cr-C-N) coatings were deposited by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS). The composition and microstructure of the coatings were investigated using electron probe micro-analysis (EPMA), x-ray photoelectron spectroscopy (XPS), glancing incident angle x-ray diffraction (GIXRD), field-emission scan electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). Mechanical and tribological properties of the coatings were measured by nanoindentation, Rockwell C test, scratch test and ball-on-disk wear tests. A correlation was pursuit between the composition, microstructure and properties of the coatings. A diagnostic of the ion species and their energy distribution generated during P-CFUBMS deposition was performed by electrostatic quadrupole plasma analyzer (EQP) to understand the improved structure and performance.When chromium and carbon targets were pulsed asynchronously in Ar and Ar+N2 atmospheres, the 52Cr+,40Ar+\12C+ and 28N2+ ion energy distributions (IEDs) were measured by EQP. The basic characteristics of these typical IEDs are similar and reproducible, which present three energy regions in all ion species:a low energy peak at 4-5 eV, a middle energy region at 20-40 eV, and a high-energy region at 90-200 eV. The reduction of duty cycle 90% to 50% led to the increase of maximum ion energy from 90 eV to 200 eV when pulsed at 100 kHz, whereas at 200 kHz, the maximum ion energy decreased firstly from 90 eV to 50 eV, and then suddenly increased to 130 eV under 50% duty cycle.The Cr-C-N coatings were deposited by pulsed direct current magnetron sputtering (PMS) under 0-200 kHz frequency and 50-90% duty cycle at 0-60% nitrogen flow rate ratio (fN2) and working pressure of 0.27 Pa. The chromium and graphite targets were respectively powered at 500-1000 W and 600-1400 W. The thickness of Cr-C-N coatings was 2-3μm and a chromium adhesion layer (about 100 nm) was firstly deposited onto the substrates. The concentration of coatings was controlled by the nitrogen flow rate ratio and the power on chromium and graphite targets, which was varied in the range of 47.0-58.9 at.%Cr,15.5-38.9 at.%C, and 36.3-55.9 at.%N. The Cr-C-N coatings consist of nanocrystalline embedded in an increased amorphous matrix as the carbon and nitrogen content were improved in the coatings. With an increase in fN2 from 20% to 50%, the atomic ratio of nitrogen to chromium (N/Cr) was increased from 0.41 to 0.92 as well as the C/Cr in the range of 0.29-0.39 when the chromium and graphite target was powered at 1000 W and 1400 W respectively. Decreasing the power on chromium target to 700 W, the N/Cr was also increased from 0.37 to 1.90, whereas the C/Cr varied in the range of 0.23-0.90 as the fN2 was increased from 20% to 50%. Under a 20% fN2 and 1000 W on chromium target, (N/Cr) in the Cr-C-N coatings was 0.54-0.62, whereas the C/Cr ratio was increased from 0.26 to 0.44 as the power on the graphite target was increased from 600 to 1000 W. The carbon occupied the sublattice positions of nitrogen in hcp-Cr2N to form the multi-compound Cr2(C,N), which transferred into fcc-Cr(C,N) with the increase of carbon and nitrogen content in the coatings. At a low chromium target power of 500 W, no crystalline phase but the Cr-containing amorphous C(N) phase was detected in the CrC1.47N1.30 coating. There were no effect of the pulse configuration on the composition and structure, which had the compositions of CrC0.23-0.35N0.83-0.91 and fcc-Cr(C,N) phase structure.A nanostructure transition from nano-columnar Cr2(C,N) or Cr(C,N) with the diameter of 20-50 nm to nanocomposite nc-Cr(C,N)/amorphous-C(N), and then to Cr-containing amorphous C(N) was detected in the Cr-C-N coatings with an increase in (C+N)/Cr from 0.81 to 2.77, which possessed the corresponding compositions of CrC0.26N0.55/CrC0.29N0.74, CrC0.35N0.91 and CrC0.90N0.88.The PMS Cr-C-N coatings exhibited a wide range of mechanical and wear resistance properties:the hardness of 11.0-30.9 GPa, the H/E ratios of 0.058-0.102, the coefficient of frictions (COF) of 0.31-0.59, the wear rates of 1.28-18.2×10-6 mm3N-1m-1, and the adhesion properties of HF1-3 and 5-23 N(LC3), which is dependent on the microstructure of the coatings. An increase of hardness from 17.5 GPa to 25.4 GPa was observed accompanied by the microstructural evolution from nano-columnar hcp-Cr2(C,N) to fcc-Cr(C,N), both of which possessed the COF of 0.52-0.56 and the low wear rates of 1.28-1.76×10-6 mm3N-1m-1. For nanocomposite nc-Cr(C,N)/amorphous-C(N) with the composition of CrC0.26N0.55, the highest hardness and H/E ratio of 30.9 GPa and 0.102 were evaluated with the lowest COF of 0.31 and the wear rate of 3.67×10-6 mm3N-1m-1 in the coatings. When the fraction of the amorphous C(N) phase increased with the further increase of carbon and nitrogen content to CrC0.29N0.74, the hardness and H/E ratio decreased to 19.7 GPa and 0.089, and the COF increased to 0.45. The poor mechanical and wear resistance properties, i.e. the lowest hardness and H/E ratio of 11.0 GPa and 0.059, and the highest wear rate of 18.20×10-6 mm3N-1m-1, were found in the Cr-C-N coating with the Cr-containing amorphous-C(N).By EQP diagnostics, the plasma generated by MPP sputtering chromium contain the ion species of 52Cr+,40Ar+,52Cr2+ and 40Ar2+ in Ar, and that of Cr1-3+, Ar1-3+, N+1-2, N2-4+, CrN+ and CrN2+ in Ar+N2, all of which exhibited a peak energy of about 3-4 eV and their intensity improved with the increase of the peak power and current. For dcMS, the deposition rate of Cr coatings increased almost linearly from 67.9 to 163.0 nm min-1 as the average power was increased from 1 to 4 kW. The deposition rates of MPP sputtered Cr coatings are in the range of 53.7-230.3 nm min-1, which are lower than the dcMS rates when the the average power is less than 2.5 kW and exceed the dcMS deposition rates as the Pd is higher than 2.5 kW. The deposition rates of MPP sputtered Cr coatings are comparable with that by dcMs, and increased almost linearly from 30 to 220 nm min-1 as an increase in the average power of 1-4 kW.The MPP Cr coatings exhibited a nano-columnar a-Cr in a diameter of 60-100 nm and possessed the hardness of 9-15 GPa, which is higher than 8.2-4.3 GPa for dcMS under the average power of 1-4 kW. A denser nano-columnar fcc-CrN microstructure showing the interruption of the columnar grain growth and a finer grain size of 45-60 nm was found in the CrN coatings deposited by MPP at 50% fN2 as compared to those of the dcMS and PMS CrN coatings. The improved microstructure in the MPP CrN coatings led to high hardness of 24.5-26 GPa, excellent wear resistance with COF of 0.33-0.36 and wear rates of 2.4-2.43 X 10-6 mm3N-1m-1. The MPP Cr-C-N coatings transited from a-Cr+13-Cr2(C,N) to Cr(C,N) with increasing the fN2 from 10% to 30%. The coatings possessed the high hardness of 23.2-24.6 GPa and the COF of 0.4-0.6. The wear rates of the coatings are in the low range of 1.2-3.7×10-6 mm3N-1m-1. The MPP technique was shown to improve the coating microstructure, and then enhance the mechanical and the wear resistance properties of the coatings.