Pulsed Closed-field Unbalanced Magnetron Sputtering Research Articles

Substrate Heating Effect on c-Axis Texture and Piezoelectric Properties of AlN Thin Films Deposited by Unbalanced Magnetron Sputtering

Aluminum nitride (AlN) thin films with highly preferred (002) orientations have been reactively deposited by a pulsed-closed field unbalanced magnetron sputtering system using TiN/Ti as the seed/adhesion layer with various substrate temperatures. The texture, orientation and piezoelectric properties of AlN films were characterized by means of x-ray diffraction, rocking curves and laser interferometry. A Michelson laser interferometer was designed and built to obtain the converse piezoelectric response of the deposited AlN thin films. It was found that a slight substrate temperature increase would significantly affect the (002) orientation and the piezoelectric coefficient of AlN thin films compared to the coating obtained with no intentional substrate heating, while higher temperature applications on substrate deteriorated the c-axis texture of the coatings without significant improvement in the piezoelectric response of AlN films.

The Development of a Nanostructured, Graded Multilayer Cr-Cr<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB>-Cr<SUB>1−<I>x</I></SUB>Al<SUB><I>x</I></SUB>N Coating Produced by Pulsed Closed Field Unbalanced Magnetron Sputtering (P-CFUBMS) for Use in Aluminum Pressure Die Casting Dies

The main objective of this research is to design an optimized 'coating system' that extends die life by minimizing premature die failure. The concept of the multilayer coating system with desired combinations of different kinds of single-layer coatings was introduced. A pulsed closed field unbalanced magnetron sputtering (P-CFUBMS) deposition system has been used to deposit Cr-CrxNy-Cr1-xAlxN compositionally graded multilayer coating structures. In this study, three power law scenarios have been adopted to vary the aluminum concentration in the graded Cr1-xAlxN layer: (i) p = 1, the aluminum concentration was increased linearly in the Cr1-xAlxN layer. (ii) p = 0.2, the Cr1-xAlxN layer is an aluminum-rich graded layer, and (iii) p = 2, the Cr1-xAlxN layer is a chromium-rich graded layer. It was found that all the graded coatings exhibit lower residual stress and higher adhesion strength than the homogeneous Cr1-xAlxN (x = 0.585) film. However, different power law grading architectures have significant influence on the hardness and wear resistance of the films. When p = 2 and p = 1, the graded films exhibited relatively low hardness values (24 and 26 GPa respectively) and high COF (0.55 to 0.60). When p = 0.2 the graded film exhibited both high hardness (34 GPa) and good wear resistance (COF = 0.45) due to the structural consistency in the graded zone. The paper discusses the correlation between the pulsing parameters and coating architecture with the resulting nanostructure and tribological properties of this Cr-CrxNy-Cr1-xAlxN coating system.

Composition and structure–property relationship of low friction, wear resistant TiN–MoS x composite coating deposited by pulsed closed-field unbalanced magnetron sputtering

Effect of MoS x content has been studied in TiN–MoS x composite coating deposited by closed-field unbalanced magnetron sputtering (CFUBMS) using separate MoS 2 and Ti target in N 2 gas environment. Pulsed dc power was applied for both the targets as well as for substrate biasing. Crystallographic orientation and structure of the coating was analysed by grazing incidence X-ray diffraction (GIXRD) technique. The surface morphology and coating fractograph were studied with field emission scanning electron microscopy (FESEM) whereas the composition of the coating was determined by energy dispersive spectroscopy (EDS) by X-ray. Scratch adhesion test, Vickers microhardness test and pin-on-disc test with cemented carbide (WC–6%Co) ball were carried out to investigate mechanical and tribological properties of the coating. Increase in MoS x content (from 6.22 wt.% to 30.43 wt.%) was found to be associated with decrease in grain size (from 63 nm to 24 nm). Maximum hardness of 32 GPa was obtained for TiN– MoS x composite coating. Film substrate adhesion was also observed to depend on MoS x content of the composite coating. Significant improvement in tribological properties was observed. With optimal MoS x content, it was possible to achieve low friction ( µ = 0.02–0.04) and wear resistant (wear coefficient = 5.5 × 10 − 16 m 3/Nm) composite solid lubricant coating.

Syntheses and characterization of TiC/a:C composite coatings using pulsed closed field unbalanced magnetron sputtering (P-CFUBMS)

TiC/a:C nanocomposite coatings were prepared by reactively sputtering titanium and graphite targets in pure argon atmosphere using a pulsed closed field unbalanced magnetron sputtering (P-CFUBMS) system. The microstructure of TiC/a:C coatings consisting of nanocrystalline TiC dispersed in an amorphous matrix of free carbon was investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The effects of coating compositions on the structure and properties of TiC/a:C coatings were investigated. In the present study, TiC/a:C coatings exhibit high hardness (24–29 GPa), low coefficient of friction (0.24–0.25) and low wear rate (less than 2.5 × 10 − 7 mm 3 N − 1 m − 1 ) when the carbon concentration is in the range of 55–66 at.%. Further increase of the carbon content beyond 70 at.% significantly decreased the volume fraction of TiC nanocrystalline and formed a large amount of free amorphous carbon in the coatings. The excessive amorphous carbon phases resulted in a decrease in the coating hardness and the sliding friction coefficient, e.g. a low COF of 0.15 was obtained when the carbon concentration reached 80.5 at.%. However, the decreased hardness will lead to an increase in the wear rate in these high carbon content TiC/a:C coatings.

Structure and properties of Ti–B–C–N nanocomposite coatings synthesized using pulsed closed field unbalanced magnetron sputtering (P-CFUBMS)

Ti-B-C and Ti–B–C–N nanocomposite coatings with various nitrogen contents were synthesized by sputtering an 80 mol.%TiB 2–20 mol.%TiC compound target in Ar/N 2 plasma using a pulsed closed field unbalanced magnetron sputtering (P-CFUBMS) system. It was found that the structure, mechanical and wear properties of Ti–B–C–N coatings exhibit a strong dependence on the nitrogen content. When the nitrogen content is in the low range, e.g. 3 at.%, the Ti–B–C–N coating was characterized as a mixture of Ti(B ,C) nanocrystallines and small volume fractions of free carbon, amorphous BN and CN x , where a super hardness of 48.5 GPa has been achieved. When the nitrogen content was increased, e.g. 10–13 at.%, the reduction of Ti(B, C) phase accompanied with the formation of nanocrystalline Ti(C,N) and amorphous BN phases were observed. The coatings can be characterized as nanocrystalline Ti(B,C) and Ti(C,N) phases imbedded in a amorphous BN, CN x and free carbon matrix, in which a slight decrease in the coating hardness (37–45 GPa), while improvements in the wear resistance were indentified. When the nitrogen content was further increased to high levels, e.g. 36 at.%, a significant increase in the volume fraction of amorphous BN and a decrease in the crystalline phases in the coatings will result in a decrease in both the hardness and the wear resistance.

A study of the oxidation behavior of CrN and CrAlN thin films in air using DSC and TGA analyses

Freestanding CrN x and Cr 1 − x Al x N films with two different Al atomic percentages with respect to the metal sublattice ( x = 0.23 and x = 0.60) were produced by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS). The dynamic oxidation behavior of the films has been characterized by thermal analysis using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structure of the films at different thermal-annealing temperatures were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) in an effort to understand different phase transitions and oxidation reactions observed on the DSC curves. The peak temperatures of the main exothermic/endothermic oxidation reactions in the DSC signals at different heating rates were applied to the Kissinger model for determination of activation energies. The mechanical properties of the films at different heat-annealing states were measured by nano-indentation. It was found that the CrN x films oxidized in air after 600 °C by the dissociation of fcc (face center cubic)-CrN to h(hexagonal)-Cr 2N and nitrogen and, after 900 °C by the dissociation of h-Cr 2N to Cr and nitrogen in the film. The addition of Al to CrN film can further improve the oxidation resistance, especially for the high temperature above 800 °C. The oxidation degradation in two Cr-Al-N films started with dissociation of fcc-CrAlN to h-Cr 2N and nitrogen in the film. The presence of thermally stable Al–N bonding in the fcc-CrAlN structure can suppress the reduction of nitrogen in the film. A dense (Cr,Al) 2O 3 layer (either amorphous or crystalline) formed at early oxidation stage (< 700 °C) can act as an effective diffusion barrier slowing down the inward diffusion of the oxygen at high temperatures. Precipitation of h-AlN phase in Cr 0.77Al 0.23N and Cr 0.40Al 0.60N films were found at 900 and 1000 °C respectively, accompanied with crystalline Al 2O 3 formation. After that, both Cr-Al-N films oxidized rapidly after the dissociation of h-Cr 2N to Cr and nitrogen. In addition, Cr 0.40Al 0.60N films exhibit higher oxidation resistance than Cr 0.77Al 0.23N films. The fcc-CrAlN was retained up to 900 °C and the precipitation of h-AlN phase took place after 1000 °C in Cr 0.40Al 0.60N films. Cr 0.40Al 0.60N films also retained a hardness of 25 GPa after annealing at 800 °C in ambient air for 1 h. The activation energies of the final oxidation exothermic peaks in CrN x , Cr 0.77Al 0.23N and Cr 0.40Al 0.60N films in the current study were found to be 2.2, 3.2 and 3.9 eV atom − 1 respectively.

Pulsed Closed Field Unbalanced Magnetron Sputtering (P-CFUBMS) Deposited TiC/a:C Thin Films

TiC/a:C nanocomposite thin film has proven to be a worthy material selection as a thin film for tribological applications due to its low coefficient of friction, good wear resistance and high hardness. In the current study TiC/a:C thin films with carbon concentration near 55-62 at % were deposited via pulsed closed field unbalanced magnetron sputtering (P-CFUBMS) in pure argon atmosphere with different substrate bias voltages and onto 440C stainless steel substrate with different substrate roughness. It was found that the TiC/a:C film hardness and elastic modulus were increased from 18.5 GPa to 33.8 GPa by increasing the substrate bias from floating to -150 V. However higher substrate bias can also decrease the film tibological properties. The substrate roughness has a strong effect on TiC/a:C film wear behavior. When the Ra (Mean surface roughness values) is less than 110 nm, the COF values are in low range (0.18-0.28). Further increase the Ra value to above 300 nm will result in a higher COF (&gt;0.33). Films deposited on higher surface roughness substrate need longer time to reach the sliding equilibrium state.

Effect of asynchronous pulsing parameters on the structure and properties of CrAlN films deposited by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS)

Controlled ion bombardment of growing thin films can be used to modify and improve the film structure and properties. Recently, higher energetic species (up to hundreds eV) were found in the plasma by pulsing the target(s) in magnetron sputtering. In this study, an electrostatic quadrupole plasma mass spectrometer (EQP) has been used in a pulsed closed unbalanced magnetron sputtering (P-CFUBMS) system to investigate the effect of different pulsing parameters (frequency and reverse time) on the ion energies and ion fluxes in the intrinsic plasma during Cr–Al–N film deposition. It is confirmed that pulsing both magnetrons in this P-CFUBMS configuration had a large effect on both the ion energies and ion fluxes generated within the plasma, which are shown to be strongly dependent on pulsing frequency and duty cycle. The effect of pulsing to provide a wide range of ion energies and ion fluxes on the film microstructure, mechanical and tribological properties was investigated using nanoindentation, microtribometry, X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (TEM). In the current study, by taking −50 V substrate bias into consideration, it was found that total ion energies with controlled pulsing parameters to achieve moderate values (70–120 eV) can effectively increase the density and decrease the grain size of Cr–Al–N films. On the other hand, pulsing regimes that produce excessive total ion energy (∼ 200 eV) result in an increase in the residual strain, and point and lattice defects in the film, which will significantly decrease the toughness and tribological properties of the film. Under optimum pulsing conditions (100 kHz and 5.0 μs), Cr–Al–N films with a dense nanostructure (column grain size of 10–40 nm) of super hardness and good wear resistance (41 GPa, 0.099 H/E ratio, 0.46 COF, and a wear rate of 3.4 × 10 − 6 mm 3N − 1 m − 1 ) have been deposited using a controlled maximum ion energy bombardment of 122 eV at high ion flux.

Effects of the substrate to chamber wall distance on the structure and properties of CrAlN films deposited by pulsed-closed field unbalanced magnetron sputtering (P-CFUBMS)

The aim of the research was to determine the dependence of the structure and properties of CrAlN films deposited by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS) on the substrate to chamber wall distance. A Hiden Electrostatic Quadrupole Plasma Analyser (EQP) has been used to investigate the positive ion energy distributions (IED) in the plasma. It was found that increasing the substrate to chamber wall distance from 127 mm to 203 mm can significantly increase the ion flux in front of the substrate. The improvement of the ion flux in front of the substrate region facilitates ion bombardment on the substrate, increases the mobility of the adatoms on the substrate surface, thereby resulting in a denser film with reduced grain size, which exhibited high hardness (∼ 30 GPa) and good wear resistance (0.35 COF, and a wear rate of 3.36 × 10 − 6 mm 3 N − 1 m − 1 ) at the substrate to chamber wall distance of 203 mm. The CrAlN film deposited at a relatively short substrate to chamber wall distance with the rotation system has comparable structure and properties with those films deposited at large substrate to chamber wall distance at fixed substrate positions.

Microstructure, mechanical and tribological properties of Cr 1− xAl xN films deposited by pulsed-closed field unbalanced magnetron sputtering (P-CFUBMS)

Nanocrystallized Cr 1− x Al x N films with various Al contents (0 to 68 at.%) were deposited by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS). The effects of aluminum content on the microstructure, mechanical and tribological properties of the Cr 1− x Al x N films have been investigated. It was found that the hardness and elastic modulus of Cr 1− x Al x N films increased with increasing Al contents in the films and reached the highest value of 36 GPa and 370 GPa, respectively, at an Al content of 58.5 at.%. Addition of Al beyond 64.0 at.% resulted in a change in crystal structure from B1 cubic to B4 hexagonal phase. The wear resistance improved gradually with the increase of Al in the Cr 1− x Al x N films. A combination of the abrasive and adhesive wear mechanism was proposed based on the SEM and EDS analysis of the wear track. The steady state dry coefficient of friction measured against a WC ball for the Cr 1− x Al x N films were in the range of 0.36–0.55, and the wear rate was in the 10 − 6 mm 3 N − 1 m − 1 range.

Examination of the pulsing phenomena in pulsed-closed field unbalanced magnetron sputtering (P-CFUBMS) of Cr–Al–N thin films

Pulsed DC magnetron sputtering was developed initially as a means of suppressing the arcing at the target during reactive sputtering. More recently, pulsing the magnetrons has also been shown to modify the intrinsic plasma parameters to bring higher ion energy and ion flux to the growing film at the substrate, which, in turn may have a strong influence on film structure and properties. This paper examines the effect of pulsing the unbalanced magnetrons in a two-cathode closed field configuration on the plasma properties, in particular, the ion energy distributions (IEDs) and ion flux, as determined using a Hiden electrostatic quadrupole plasma mass spectrometer (EQP). Chromium and aluminum targets were reactively sputtered in an Ar–N 2 atmosphere in this two-cathode pulsed-closed field unbalanced magnetron sputtering (P-CFUBMS) system. The pulsing parameters examined included: pulsed power (400 W to 1400 W), working pressure (0.13 Pa/1.0 mTorr to 0.53 Pa/4 mTorr), nitrogen to argon flow ratio, substrate to target distance (127 mm/5 in. and 203 mm/8 in.), duty cycle (40 kHz to 350 kHz frequency, 0.4 μs to 5 μs reverse time), operated under synchronized (voltage-time wave forms of the two magnetrons were in phase) and asynchronized (voltage-time wave forms of the two magnetrons were out of phase) conditions. The effect of the pulsing regimes on the ion energy and ion flux was determined for reactive sputtering of Cr–Al–N thin films. It was shown that pulsing both magnetrons in this P-CFUBMS configuration had a large effect on both the ion energies and ion fluxes generated within the plasma. The positive voltage overshoot and the positive pulse voltage developed on each target during the reverse voltage (positive) period increased the plasma potential and thereby increased the overall extended ion energy range. Three main ion energy regions in the IEDs were identified and related to different positive voltage values during the reverse pulse period. In addition, the extent of synchronization of the voltage waveforms of the two magnetrons also substantially influenced both the IEDs and the ion fluxes, while varying the pulsing frequencies and reverse times (duty cycle) in either synchronized or asynchronized modes also resulted in different ion energies and ion fluxes within the plasma. It was also found that the ion flux increased on increasing the EQP orifice to the chamber wall distance from 127 mm to 203 mm, and the ion energy range decreased as the working pressure increased from 0.13 Pa to 0.53 Pa. The ion energies and fluxes are shown to be strongly dependent on pulsing frequency, duty cycle and waveform.