Pulsed Closed Field Unbalanced Magnetron 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.

Seed layer effect on the (0 0 2) texture and piezoelectric properties of pulsed-DC sputtered AlN thin films

Piezoelectric aluminium nitride (AlN) thin films were deposited by reactive sputtering of Al metal target in nitrogen atmosphere using a pulsed closed field unbalanced magnetron sputtering system on various substrates using different underlayers (seed layer) such as Pt and Mo. The texture, orientation and piezoelectric properties of AlN films were characterised by means of X-ray diffraction, field emission scanning electron microscopy and laser interferometry. A Michelson laser interferometer was designed to obtain the converse piezoelectric response of the deposited AlN thin films. It was found that the incorporation of different seed layers at various working pressure in the sandwiched structure, significantly affected the (0 0 2) orientation and the piezoelectric response of AlN thin films.

The Process Optimization of Smart Nanostructured AlN Thin Films Sputtered by Pulsed DC

A ‘Smart’ coating design, based on the incorporation of an AlN piezoelectric thin film sensor into a tribological coating system, has been explained in this paper. As part of the overall coating system, piezoelectric AlN thin films with highly preferred (002) orientations have been deposited by a pulsed closed field unbalanced magnetron sputtering system using Cr, Mo and Pt as the seed/adhesion layer. The texture, orientation and piezoelectric properties of AlN films were characterized by means of x-ray diffraction, field emission scanning electron microscopy and laser interferometry. A Michelson laser interferometer was designed and constructed to obtain the converse piezoelectric response of the deposited AlN thin films. It was found that the incorporation of different seed layers at various working pressure in the sandwiched structure, significantly affected the (002) orientation and the piezoelectric response of AlN thin films.

Nanostructure transition in Cr–C–N coatings deposited by pulsed closed field unbalanced magnetron sputtering

Cr–C–N coatings with different compositions, i.e. (C+N)/Cr atomic ratios (x) of 0.81–2.77, were deposited using pulsed closed field unbalanced magnetron sputtering by varying the chromium and graphite target powers, the pulse configuration and the ratio of the nitrogen flow rate to the total gas flow rate. Three kinds of nanostructures were identified in the Cr–C–N coatings dependent on the x values: a nano-columnar structure of hexagonal closed-packed (hcp) Cr2(C,N) and face-centered cubic (fcc) Cr(C,N) at x=0.81 and 1.03 respectively, a nanocomposite structure consisting of nanocrystalline Cr(C,N) embedded in an amorphous C(N) matrix at x=1.26 and 1.78, and a Cr-containing amorphous C(N) structure at x=2.77. A maximum hardness of 31.0GPa and a high H/E ratio of 1.0 have been achieved in the nc-Cr(C,N)/a-C(N) nanocomposite structure at x=1.26, whereas the coating with a Cr-containing amorphous C(N) structure had a minimum hardness of 10.9GPa and a low H/E ratio of 0.08 at x=2.77. The incorporation of carbon into the Cr–N coatings led to a phase transition from hcp-Cr2(C,N) to fcc-Cr(C,N) by the dissolution into the nanocrystallites, and promoted the amorphization of Cr–C–N coatings with the precipitation of amorphous C(N). It was found that a high x value over 1.0 in the Cr–C–N coatings is the composition threshold to the nanostructure transition.

TiBCN:CNx multilayer coatings deposited by pulsed closed field unbalanced magnetron sputtering

In this study, a combination of nanocomposite and multilayer coating design was investigated in an effort to reduce the coefficient of friction (COF) while maintaining good mechanical properties of the TiBCN coatings. The TiBCN:CNx coatings consist of TiBCN and CNx nanolayers which were deposited alternately by reactive sputtering a TiBC composite target (80mol% TiB2+20mol% TiC) and a graphite target in an Ar:N2 mixture using a pulsed closed field unbalanced magnetron sputtering system. Low angle X-ray diffraction and transmission electron microscopy characterizations confirmed that the coatings consist of different bilayer periods in a range of 3.5 to 7.0nm. The TiBCN layers exhibited a nanocomposite structure, whereas the CNx layers were in an amorphous state. The mechanical properties and wear resistance of the TiBCN:CNx multilayer coatings were investigated using nanoindentation and ball-on-disk wear test. The TiBCN:CNx coatings exhibited high hardness in a range of 20–30GPa. The highest hardness of 30GPa was achieved in the coating with a bilayer period of 4.5nm. A low COF of 0.17 sliding against a WC-Co ball was obtained at a bilayer period of 4.5nm, which is much lower than those of the single layer TiBCN and TiBC nanocomposite coatings (0.55–0.7).

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.

Microstructure, mechanical and tribological properties of Cr–C–N coatings deposited by pulsed closed field unbalanced magnetron sputtering

Nanocrystalline Cr–C–N coatings were deposited by pulsed closed field unbalanced magnetron sputtering. The microstructure, chemical composition and elemental chemical state of the coatings were investigated using field-emission scanning electron microscopy, glancing incident angle x-ray diffraction and x-ray photoelectron spectroscopy. Mechanical and tribological properties of the coatings were measured by nanoindentation and ball-on-disk wear tests. The Cr–C–N coatings consist of nanocrystalline Cr 7C 3, CrN embedded in an amorphous carbon and CN matrix. The coatings with a N content of 25.5 at.% possessed a highest hardness of 25.4 GPa and a highest H/E ratio of 0.096. Dependent on the C content, the steady state dry coefficient of friction values for Cr–C–N coatings sliding against a WC-Co ball were found in the range of 0.38–0.56. The wear rates of the coatings are in the low range of 1.28–3.44 × 10 − 6 mm 3 N − 1 m − 1 .

Nano-structured CrN/AlN multilayer coatings synthesized by pulsed closed field unbalanced magnetron sputtering

CrN/AlN superlattice coatings with bilayer period of 2.5 to 22.5 nm were prepared using a pulsed closed field unbalanced magnetron sputtering system. The Al/(Cr + Al) ratios of the coatings were in a range of 61.5%–66.5%. It was found that the AlN layers exhibit Wurtzite type structure when the AlN layer thickness is larger than 3.3 nm. The structure difference between CrN and AlN layers led to low hardness (23–25 GPa), poor adhesion, and low wear resistance of the coatings. The CrN layers epitaxially stabilize the AlN layers to NaCl-type structure as the thickness of the AlN layer is small enough (≤ 3.3 nm). The NaCl type CrN/AlN coatings exhibit super hardness above 40 GPa at Λ of 3.0–4 nm. The highest hardness of 45 GPa was achieved at Λ = 3 nm. The coatings with Λ of 3.0–4.7 nm also showed improved adhesion, toughness, and excellent wear resistance, in which a low coefficient of friction of 0.32 and excellent wear rates in the low 10 − 7 × mm 3N − 1 m − 1 range were achieved. To avoid the formation of Wurtzite type AlN structure in CrN/AlN superlattice coatings, the AlN layer thickness should be carefully controlled (e.g. ≤ 3.3 nm in the present study).

CrN/AlN superlattice coatings synthesized by pulsed closed field unbalanced magnetron sputtering with different CrN layer thicknesses

CrN/AlN superlattice coatings with different CrN layer thicknesses were prepared using a pulsed closed field unbalanced magnetron sputtering system. A decrease in the bilayer period from 12.4 to 3.0 nm and simultaneously an increase in the Al/(Cr + Al) ratio from 19.1 to 68.7 at.% were obtained in the CrN/AlN coatings when the Cr target power was decreased from 1200 to 200 W. The bilayer period and the structure of the coatings were characterized by means of low angle and high angle X-ray diffraction and transmission electron microscopy. The mechanical and tribological properties of the coatings were studied using the nanoindentation and ball-on-disc wear tests. It was found that CrN/AlN superlattice coatings synthesized in the current study exhibited a single phase face-centered cubic structure with well defined interfaces between CrN and AlN nanolayers. Decreases in the residual stress and the lattice parameter were identified with a decrease in the CrN layer thickness. The hardness of the coatings increased with a decrease in the bilayer period and the CrN layer thickness, and reached the highest value of 42 GPa at a bilayer period of 4.1 nm (CrN layer thickness of 1.5 nm, AlN layer thickness of 2.5 nm) and an Al/(Cr + Al) ratio of 59.3 at.% in the coatings. A low coefficient of friction of 0.35 and correspondingly low wear rate of 7 × 10 − 7 mm 3N − 1 m − 1 were also identified in this optimized CrN/AlN coating when sliding against a WC-6%Co ball.

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.

Investigation of processing parameters for pulsed closed field unbalanced magnetron sputtered titanium carbide thin films

Titanium carbide is a well established wear resistant coating owing to its excellent tribological properties including high hardness and elastic modulus, good wear resistance, low coefficient of friction against steel and high temperature stability. Recent advances in sputtering technology have resulted in improvements in the properties and performance of wear resistant coatings. Closed field unbalanced magnetron sputtering and pulsed magnetron sputtering have greatly improved the structure and properties of titanium carbide films by increasing ion bombardment at the substrate. However, film quality is highly dependent on the placement of the substrate relative to the sputtering targets. Titanium carbide films were deposited using a cosputtering configuration from one graphite target and one titanium target. At short substrate to target distances films exhibited an open, porous structure with low hardness and a high coefficient of friction. As the substrate to target distance was increased, along with the angle of incident particles, improvements were seen in both the microstructure and tribological properties.

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.