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Journal ArticleDOI

α-alumina coatings on WC/Co substrates by physical vapor deposition

T. I. Selinder, E. Coronel, Erik Wallin1, Ulf Helmersson1 
01 Mar 2009-International Journal of Refractory Metals & Hard Materials (Elsevier)-Vol. 27, Iss: 2, pp 507-512

AbstractPhysical vapor deposition coatings for cutting tools may be deposited by, e.g. reactive magnetron sputtering. Alumina growth in Ar/O2 gas mixtures gives rise to problems due to insulating layers on ...

Summary (1 min read)

Introduction

  • Hard physical vapor deposited (PVD) coatings may be coated by a variety of methods, e.g. direct current (DC) magnetron sputtering in Ar inert gas.
  • Albeit stable, BPDMS has only been repeatedly successful in depositing γ-alumina, which is a drawback if phase stability of the coating is of concern.
  • So far, commercial α-alumina coatings have only been grown by chemical vapor deposition (CVD) at relatively high substrate temperatures.
  • Therefore one drawback with HiPIMS has been the inherently low growth rate.
  • Recently, it was shown that high rate sputtering is possible, and that long time stable operation of Ar/O2 discharges is possible without feedback control of the reactive gas [12].

Experimental details

  • Coatings were made in a laboratory scale ultra-high vacuum system, with a base pressure of less than 3×10-5 Pa.
  • Cemented carbide, WC10%Co (H10F), substrates were mounted flat on a resistive heater directly above the magnetron, at a distance of 11 cm.
  • The temperature reported herein is always the set nominal temperature, however, not the estimated temperature.
  • To the extent of the analyses performed in the present work the process was reproducible, in that consecutive runs under the Page 4 of 18 same conditions yielded coatings with similar properties.
  • TEM cross-section samples were produced by means of classical sample preparation by first mechanical dimpling with diamond particles and finally thinned to electron transparency by Ar ion etching.

On the HiPIMS Process

  • HiPIMS resembles ordinary magnetron sputtering in many aspects.
  • In many cases the sputtering yield is lower for the compound now covering the target, and the deposition rate drops.
  • These coatings were deposited at 650ºC nominal temperature, but it is estimated that the real extending almost throughout the total film thickness.
  • XRD of this particular sample did, however, not indicate anything else but a pure α-alumina coating, a fact probably due to a very small γ-grain size.
  • Since it is anticipated that alumina will improve the chemical stability of the tool surface on which the chips flow a test was selected that emphasizes the wear on the rake face.

Conclusions

  • In this study the HiPIMS technique was used to deposit alumina coatings on WC/Co substrates and tools.
  • Α-alumina was succesfully deposited on WC/Co substrates at 650 °C. .
  • The deposition rate was found to be independent of substrate bias and substrate temperature in the range of 500 to 650 °C. .
  • The crystalline structure depended on deposition temperature.
  • Hence, these first tests are promising in terms of the potential for using HiPIMS deposited alumina as a wear-resistant coating in metal cutting applications.

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Linköping University Post Print
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α-alumina coatings on WC/Co substrates by
physical vapor deposition
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T.I. Selinder, E. Coronel, Erik Wallin and Ulf Helmersson
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N.B.: When citing this work, cite the original article.
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Original Publication:
T.I. Selinder, E. Coronel, Erik Wallin and Ulf Helmersson, α-alumina coatings on WC/Co
substrates by physical vapor deposition, 2009, International journal of refractory metals
& hard materials, (27), 2, 507-512.
http://dx.doi.org/10.1016/j.ijrmhm.2008.10.007
Copyright: Elsevier Science B.V., Amsterdam.
http://www.elsevier.com/
Postprint available at: Linköping University Electronic Press
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-15359

α-ALUMINA COATINGS ON WC/Co SUBSTRATES BY PHYSICAL VAPOR
DEPOSITION
T.I. Selinder*, E. Coronel*, E. Wallin**, U. Helmersson**
*Sandvik Tooling, SE-126 80 Stockholm, Sweden
**Plasma and Coatings Physics Division, IFM Material Physics, Linköping University,
SE-581 83 Linköping, Sweden
Abstract
Physical vapor deposition coatings for cutting tools may be deposited by, e.g. reactive
magnetron sputtering. Alumina growth in Ar/O
2
gas mixtures gives rise to problems due
to insulating layers on targets, and hysteresis effects with respect to oxygen gas flow. In
this paper is described a technology for the deposition of crystalline alumina: reactive
high power impulse magnetron sputtering. Pure Al was used as target material, and the
cemented carbide (WC/Co) substrates were kept at 500-650ºC. Hysteresis effects with
respect to oxygen gas flow were alleviated, which enabled stable growth at a high
deposition rate. The high power impulses were helpful in obtaining a crystalline oxide
coating. X-ray diffraction and cross-section transmission electron microscopy showed
that α-alumina films were formed. Technological testing of these PVD alumina
coatings, with state-of-the-art AlTiN as benchmark, showed significantly improved
crater wear resistance in steel turning.
Keywords: HiPIMS, HPPMS, ionized-PVD, alumina, corundum
Page 1 of 18

Introduction
Hard physical vapor deposited (PVD) coatings may be coated by a variety of methods,
e.g. direct current (DC) magnetron sputtering in Ar inert gas. Reactive sputtering in
Ar/oxygen gas mixtures, may be used to grow oxides, but for insulators, in particular
alumina, this gives rise to problems due to the formation of insulating layers.
Conducting surfaces are a prerequisite to sustain an electrical glow discharge in DC
mode. These problems are circumvented in the bipolar pulsed magnetron technique
(BPDMS) [1-2], in which two magnetrons work in pair, alternating as anode and
sputtering target. By using pulse frequencies in the range of a few kHz the Al erosion
rate exceeds the rate of oxide formation at each target surface. Albeit stable, BPDMS
has only been repeatedly successful in depositing γ-alumina, which is a drawback if
phase stability of the coating is of concern. So far, commercial α-alumina coatings have
only been grown by chemical vapor deposition (CVD) at relatively high substrate
temperatures. In order to coat heat sensitive substrates, e.g. polycrystalline c-BN, high
speed steel, brazed tools, etc. the process temperature needs to be reduced. For this and
other reasons research and development on PVD alumina by, for instance, dual
magnetron sputtering [3], and arc evaporation [4] has been intense for the past few
years. Preoxidation of Cr-containing surfaces have enabled nucleation and growth of α-
alumina [5], but still requires substrate temperatures of 700-750ºC. A commercial
technology capable of growing highly crystalline mixed oxides (Al,Cr)
2
O
3
by the arc
method at temperatures below 600ºC were recently made available on the market [6],
but the commercialization of pure PVD α-alumina still remains. In this context the
novel technology high power impulse magnetron sputtering (HiPIMS) [7,8]have been
studied because this intermittent very high energetic sputtering technique widens the
Page 2 of 18

possibilities of supplying energy to the growth surface, which likely is the key to
success in nucleating α-alumina, and sustaining its growth. The HiPIMS technique has
been studied for deposition of both conducting coatings [9] as well as for oxide coatings
[10-11]. The degree of ionization of the sputtered material may be higher than for
traditional sputtering but the ionized species also accelerate towards the target, so-called
self sputtering, that has a lower sputtering yield than that of Ar sputtering. Therefore
one drawback with HiPIMS has been the inherently low growth rate. Recently, it was
shown that high rate sputtering is possible, and that long time stable operation of Ar/O
2
discharges is possible without feedback control of the reactive gas [12]. Stoichiometric
alumina was grown, with a phase structure that was dependent on substrate temperature;
at temperatures as low as 650ºC α-alumina was the only crystalline phase detected by
X-ray diffraction (XRD). Initial transmission electron microscopy (TEM) studies
revealed, however, that γ-alumina may grow, at least during the early stages in film
growth directly on WC/Co. XRD indicates that at the same temperature and identical
growth conditions AlTiN pre-coated substrates tend to favor growth of α- over γ-
alumina. A possible origin of this difference is discussed.
Experimental details
Coatings were made in a laboratory scale ultra-high vacuum system, with a base
pressure of less than 3×10
-5
Pa. A planar, 50mm diameter shuttered magnetron
sputtering source was mounted with a pure (99.999%) 3mm thick Al target. Cemented
carbide, WC10%Co (H10F), substrates were mounted flat on a resistive heater directly
above the magnetron, at a distance of 11 cm. Temperatures during different runs were
maintained at 650, 575, and 500 ºC. In order to grow films on cutting edges ISO style
Page 3 of 18

CNMG120408-MM H10F substrates were mounted, two at a time, on a tilted holder.
One nose and cutting edge was directed towards the magnetron. In this way both the
tool rake and flank faces were coated. In the latter case, however, it is estimated that the
real substrate temperature is approx. 50ºC less than the nominal set value. The
temperature reported herein is always the set nominal temperature, however, not the
estimated temperature. Substrates were either floating, or bias was applied by radio
frequency (RF) by an Advanced Energy RFX-600 power supply.
A fixed 99.997% pure Ar flow was adjusted to establish a pressure of 2.7 Pa. The
substrates were sputter etched for 15 minutes in an RF glow discharge resulting in a
-175 V DC bias of the sample holder, after which the pressure was reduced to 0.8 Pa,
and the sputtering initiated with the shutter closed. Sputtering was carried out by means
of a DC voltage source pulsed by a Melec SPIK1000A unit. The Al target was sputtered
at an average power of 110W (~5 Wcm
-2
) in pulses having a peak power in excess of
6kW, or 300 Wcm
-2
. The process working point was controlled by selecting the
99.9995% pure oxygen flow, to a value of approximately 2% of the Ar flow. The
oxygen flow was determined empirically so that transparent coatings were deposited. It
is expected that substrate oxidation during this stage is low: As the oxygen was
introduced when the discharge was running with the shutter closed it is reasonably
assumed that the partial pressure of oxygen is low in the deposition chamber; in the
working point chosen it is to a large extent consumed, incorporated in the growing film.
Also, there were no visible discolorations, after coating runs, of uncoated substrate
surfaces exposed to the sputtering atmosphere. To the extent of the analyses performed
in the present work the process was reproducible, in that consecutive runs under the
Page 4 of 18

Citations
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Abstract: Cr2O3 and (Cr,Al)(2)O-3 films were grown using reactive dc and inductively coupled plasma magnetron sputtering at substrate temperatures of 300-450 degrees C. For pure chromia, alpha-Cr2O3 films with fiber texture were grown; the out-of-plane texture could be controlled from andlt; 0001 andgt; to andlt;10andlt;(1)over barandgt;4andgt;. The former texture was obtained as a consequence of competitive growth with no applied bias or inductively coupled plasma, while the latter was obtained at moderate bias ( - 50 V), probably due to recrystallization driven by ion-bombardment-induced strain. By reactive codeposition of Cr and Al, a corundum-structured metastable solid solution alpha-(Cr,Al)(2)O-3 with Cr/Al ratios of 2-10 was grown with a dense, fine-grained morphology. Hardness and reduced elastic modulus values were in the ranges 24-27 GPa and 190-230 GPa, respectively.

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References
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Journal ArticleDOI
Abstract: In plasma-based deposition processing, the importance of low-energy ion bombardment during thin film growth can hardly be exaggerated. Ion bombardment is an important physical tool available to materials scientists in the design of new materials and new structures. Glow discharges and in particular, the magnetron sputtering discharge have the advantage that the ions of the discharge are abundantly available to the deposition process. However, the ion chemistry is usually dominated by the ions of the inert sputtering gas while ions of the sputtered material are rare. Over the last few years, various ionized sputtering techniques have appeared that can achieve a high degree of ionization of the sputtered atoms, often up to 50% but in some cases as much as approximately 90%. This opens a complete new perspective in the engineering and design of new thin film materials. The development and application of magnetron sputtering systems for ionized physical vapor deposition (IPVD) is reviewed. The application of a secondary discharge, inductively coupled plasma magnetron sputtering (ICP-MS) and microwave amplified magnetron sputtering, is discussed as well as the high power impulse magnetron sputtering (HIPIMS), the self-sustained sputtering (SSS) magnetron, and the hollow cathode magnetron (HCM) sputtering discharges. Furthermore, filtered arc-deposition is discussed due to its importance as an IPVD technique. Examples of the importance of the IPVD-techniques for growth of thin films with improved adhesion, improved microstructures, improved coverage of complex shaped substrates, and increased reactivity with higher deposition rate in reactive processes are reviewed.

872 citations


Journal ArticleDOI
Abstract: Using a novel pulsed power supply in combination with a standard circular flat magnetron source, operated with a Cu target, a peak power density of 2800 W cm -2 was achieved. This results in a very intense plasma with peak ion current densities of up to 3.4 A cm −2 at the substrate situated 10 cm from the target. The ionized fraction of the deposited Cu flux was estimated to be approximately 70% from deposition rate measurements. The potential for high-aspect-ratio trench filling applications by high power pulsed magnetron sputtering is demonstrated by deposition in via-structures. The high power pulsed technique also results in a higher degree of target utilization and an improved thickness uniformity of the deposited films compared with conventional d.c. magnetron sputtering.

868 citations


Journal ArticleDOI
Abstract: Reactive sputtering is a commonly used process to fabricate compound thin film coatings on a wide variety of different substrates. The industrial applications request high rate deposition processes. To meet this demand, it is necessary to have very good process control of such processes. The deposition rate is extremely sensitive to the supply of the reactive gas. A too low supply of the reactive gas will cause high rate metallic sputtering, but may give rise to an understoichiometric composition of the deposited film. A too high supply of the reactive gas will allow for stoichiometric composition of the deposited film, but will cause poisoning of the target surface, which may reduce the deposition rate significantly. This behaviour points out that there may exist optimum processing conditions where both high rate and stoichiometric film composition may be obtained. The purpose of this article is to explain how different parameters affect the reactive sputtering process. A simple model for the reactive sputtering process is described. Based on this model, it is possible to predict the processing behaviour for many different ways of carrying out this process. It is also possible to use the results of the modeling work to scale processes from laboratory size to large industrial processes. The focus will be to obtain as simple a model that will still quite correctly describe most experimental findings. Despite some quite crude approximations, we believe that the model presented satisfies this criterion.

572 citations


Journal ArticleDOI
Abstract: The method of reactive gas control during reactive sputtering strongly influences the deposition rate and film properties of the compound being deposited. Flow control of the reactive gas is the simplest method, but since reactive sputtering is typically done in the compound or poisoned mode of the target, the deposition rate is low compared to the rate from the elemental target. In addition, the film properties produced by flow control reactive sputtering are less than optimal. Partial pressure control of the reactive gas is more complex than flow control because it requires active feedback control, but it allows operation of the process in the transition region between the elemental and poisoned states of the target. By operating in this region, higher deposition rates compared to flow control are achieved, and the film properties are improved. Reactive sputtering of insulating films requires the use of the right type of power to prevent arcing on the target, which is detrimental to the quality of the deposited films. Both pulsed dc and mid-frequency ac power prevent arcing during the reactive sputtering of insulating films. Arc prevention eliminates droplet ejection from the target and allows the reactive deposition to occur without large fluctuations in the reactive gas partial pressure, often a result of uncontrolled arcing. Ternary or quaternary compounds can be reactively sputtered using dual magnetron set-ups and multiple reactive gases. Multiple reactive gases present an additional control problem in that one of the reactive gases can trap the target in the poisoned mode unless the partial pressures of both reactive gases are individually controlled. Reactive sputtering with high power pulsed magnetron sputtering, which provides a high degree of ionization of the sputtered species, is possible for insulating films as long as the partial pressure of the reactive gas is controlled and arc detection and suppression is available on the power supply.

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Abstract: The commonly used current-voltage characteristics are found inadequate for describing the pulsed nature of the high power impulse magnetron sputtering (HIPIMS) discharge; rather, the description needs to be expanded to current-voltage-time characteristics for each initial gas pressure. Using different target materials (Cu, Ti, Nb, C, W, Al, and Cr) and a pulsed constant-voltage supply, it is shown that the HIPIMS discharges typically exhibit an initial pressure dependent current peak followed by a second phase that is power and material dependent. This suggests that the initial phase of a HIPIMS discharge pulse is dominated by gas ions, whereas the later phase has a strong contribution from self-sputtering. For some materials, the discharge switches into a mode of sustained self-sputtering. The very large differences between materials cannot be ascribed to the different sputter yields but they indicate that generation and trapping of secondary electrons play a major role for current-voltage-time characteristics. In particular, it is argued that the sustained self-sputtering phase is associated with the generation of multiply charged ions because only they can cause potential emission of secondary electrons, whereas the yield caused by singly charged metal ions is negligibly small.

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Frequently Asked Questions (1)
Q1. What have the authors contributed in "Α-alumina coatings on wc/co substrates by physical vapor deposition" ?

In this paper, high power impulse magnetron sputtering ( HiPIMS ) was used to deposit alumina coatings on WC/Co substrates and tools.