E L S E V I E R Surface and Coatings Technology 74-75 (1995) 497 502

PVD coatings on aluminium substrates

E. Lugscheider, G. Kr~imer 1, C. Barimani, H. Zimmermann

Aachen University of Technology, Lehr-und Forschungsgebiet Werkstoffwissenschaften, Templergraben 55, 52056 Aachen. Germany

Abstract

It is well known that aluminium is today the most widely used metallic material besides steel. The mechanical characteristics of aluminium offer an increasing application field, especially where lightweight constructions are required. The demands for improved characteristics such as higher strength and greater durability are achieved by the development of new aluminium alloys. Continuous efforts are made in research into new possibilities for making use of the advantages of aluminium in applications that were reserved up to now for harder and more wear-resistant materials. Because of their environmental benefits modern PVD processes represent a better alternative to a number of conventional coating processes to deposit wear-resistant films on aluminium surfaces.

The aim of this paper is to describe the possibilities of TiN coating on an aluminium alloy, A1MgSil, by application of the arc- PVD process without damage to the heat-sensitive substrate material. Different machining and cleaning processes were used for the preparation of the aluminium substrates. The relationship between substrate pretreatment and coating characteristics is demonstrated. The influence of different process parameters such as etching, bias voltage and coating time as well as the arrangement of the substrates in the vacuum chamber were investigated. Multilayer (Ti and TiN) and graded coatings were deposited on the aluminium alloy to achieve the best results in deposition rate, microhardness and critical scratch load. To characterize the abrasive wear behaviour of the coated aluminium substrates a special sandblasting test was used. These results are also described in this paper. Finally a suitable process technology is provided for the application of thin protective films on aluminium in order to increase wear resistance.

Keywords: PVD coatings; Aluminium substrates; Wear resistance; Sandblasting test; Heat treatment

1. Introduct ion

During the deposition of wear-resistant PVD hard coatings on aluminium, several problems arise owing to a lack of support and the low melting point of the substrate material, and also the immediate formation of oxide top layers. The solution to these problems requires some fundamental knowledge of the material aluminium.

High-purity aluminium (purity: 99.999% A1) is an unsuitable material for construction, owing to a tensile yield strength of 2 0 N m m -2 and a recrystallization temperature from - 60 to - 40 °C, making strengthening by cold working impossible. An increase in accompany- ing elements such as Fe, Si and Ti of 0.01% raises the recrystallization temperature by more than 200 °C. With decreasing purity in a soft annealed state, the tensile yield strength, proof limit and hardness increase to values of Rpo.2-25 N mm -2, Rm=70 N m m -2 (purity: 99.5% A1), while the elongation at rupture decreases to

1 Present address: McKinsey & Company, Inc., D0sseldorf, Germany.

0257-8972/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0257-8972(95 )08305-7

values of Alo=50%. The reasons for this increase in toughness are the fine-grained structure, which is especi- ally caused by small contents of titanium and boron, and the increase in toughness due to elements dissolved in the mixed crystal, where copper, iron and silicon yield the biggest increase in strength. A further increase in toughness can be achieved through cold working [ 1].

The corrosion resistance of aluminium in particular meets high requirements and is further improved through higher degrees of purity. Aluminium in the atmosphere becomes coated, through reaction with oxygen and steam, with a thin but dense natural oxide layer. If this layer is mechanically damaged or chemically removed, new formation of alumina starts spontaneously (self- healing effect). The oxide layer consists mainly of amorphous A1203. Oxides of some alloy elements can be incorporated into the alumina layer and may improve the properties of this natural oxide layer within certain limits, e.g. magnesium oxide. In contrast to the oxide layers of many other metals this thin, the dense and isolating oxide layer of aluminium exhibits very good adhesion and therefore provides a secure protection

498 E. Lugscheider et al./'SurJace and Coatings Technology 74 75 (1995) 497 502

from further oxidation of the metal underneath. This property is the reason for the superior resistance against atmospheric effects and numerous inorganic and organic materials [2].

The oxide layer on polished aluminium in dry air at room temperature reaches a thickness of 0.001 ~tm in a few minutes. Within some days it grows with decreasing growth rate to about two or three times this thickness. Higher temperatures (e.g. heat treatment) accelerate and increase the growth rate of this natural oxide layer. This oxide layer can grow in a humid atmosphere under formation of hydroxide to more than 0.1 tam [1,2].

For PVD coatings this oxidation behaviour of alumin- ium may cause problems. Investigations of different aluminium samples, which were exposed for various times to different atmospheric conditions, revealed that mirror polishing just prior to the deposition process is a suitable preparation for the arc-PVD coating process used. Especially influenced by the metal ion etching, the existing oxide layers show no adverse influence on the adhesion of the coating. If the samples are exposed for too long to a warm and humid atmosphere, coating chip-off has to be expected.

Aluminium forms hardenable alloys with many alloy- ing elements. The toughness values of these alloys can be improved through an adjusted heat treatment, i.e. precipitation hardening, taking into consideration the special solubility effects. This heat treatment is divided into the following three steps: (1) Solution heat treatment. Through annealing at high

temperature (below the melting temperature of the phase with the lowest melting temperature) there is as much as possible of the alloying components dissolved in the aluminium solid solution, leading to the precipitation hardening effects.

(2) Rapid cooling~quenching. Through quenching (e.g. in water) the solid solution, enriched with alloys, is transferred into an supersaturated condition.

(3) Aging. Through aging (at room temperature or moderate temperature) precipitation from the super- saturated solid solution takes place, leading to an increase of tensile strength, the 0.2% proof limit and hardness. These precipitations exist for natural aging (at room temperature) coherently within the solid solution, while at moderate aging temperatures some metastable phases occur which are only partly coher- ent with the solid solution. With this so-called artificial aging (100-200 °C) the strength properties rise more rapidly and show higher peak strengths. Aging at too high temperatures as well as too long aging times leads to the formation of incoherent phases and therefore a decrease in strength (over- aging) [ 1,2].

The alloy A1MgSil used for the present coating experi- ments is used in many fields (containers, pipes, wrought parts, aeroplane components, decorative parts for cars,

truck constructions, hulls, sailing boat masts, machine bodies, casings, welding constructions, food processing [ 1 ]). This alloy is both naturally and artificially precipi- tation heat treatable, and the aging effect is due to the precipitate Mg2Si. The highest strength values are reached with artificial aging, therefore this is preferen- tially used. The influence of the temperature of the solution heat treatment at 520 540 r~C is shown in Fig. 1 [-2]. The hardness values vs. aging time are given in Fig. 2. Measurement of Brinell hardness, HB, (German Industry Standard DIN 50 351) was not practicable, therefore Vickers hardness Hv was measured according to German Industry Standard DIN 50 133. The Hv hardness values are about 15% higher than the HB values for aluminium [3].

An improvement of the wear properties of aluminium through a TiN hardcoating is only possible if there is a sufficient hardness of the substrate material. For a sub- strate that is too soft the risk exists that the hard TiN coating is pressed under load into the substrate and chipping-off occurs as a result of significant plastic deformation, as illustrated in Fig. 3. For heat-treatable aluminium alloys there are basically two possibilities to obtain the maximum substrate hardness during a PVD coating process. First, an alloy can be chosen that shows a heat treatment characteristic similar to the temperature of the coating process. This allows the aging process to be conducted, following the quenching, during the coat-

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aging time

Fig. 1. Different properties of AIMgSil depending on aging temper- ature and aging time (annealing temperature: 520 °C, water quench).

E. Lugscheider et al./Surface and Coatings Technology 74 75 (1995) 497-502 499

120

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heat treatment of AIMgSil .. annealing temperature: 540°C Z l annealing time: 2h : : water quench : : aging temperature: 160°C ii

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Fig. 2. Hardness of AIMgSil depending on aging time.

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rolls show that the formation of mixed oxides improves coating adhesion [4]. Pan also mentions a gradient interlayer containing the intermetallic phases FeTiO 3 and TiO2, improving adhesion, for the coating of steel with TiC [5]. Unfortunately, the TiC coating lifts off from the TiO2 coating, since the tetragonal TiO2 and the cubic TiC have only a limited solubility. Sputtered, refractory TiC coatings show especially good adhesion on aluminium if a mixed compound is formed consisting of coating material, oxide layer and substrate material. The adhesion may be further improved through adding nitrogen [6]. Coad [7] found that a 3 nm thick oxide layer prevents the formation of a brittle intermetallic phase in the interface of TiC and aluminium.

Fig. 3. Spalled TiN coating on plastically deformed aluminium.

ing procedure. The second possibility, which was used here, is to keep the coating temperatures after the aging as low as possible in order not to affect the strength properties adversely. Especially for artificially aging alloys the coating process temperatures have to be significantly below the aging temperatures. The PVD technology offers some possibilities, which were investi- gated here.

The oxide layers forming as a result of the oxygen affinity of aluminium could have adverse affects on the bond strength of hardcoatings since they form a dense, inert surface boundary and diffusion processes or other interactions between coating and substrate material are prevented. In contrast to this, experiences of aluminium, titanium or chromium coatings on partly oxidised steel

2. Experimental details

For the coating experiments round AIMgSil samples with a diameter of 20 mm and a height of 5 mm were used. The aluminium samples were artificially aged, i.e. they were solution heat treated at 540°C for 2h, quenched in water and aged at 160 °C for 24 h. For the surface treatment of the samples a lapping machine was used. Just prior to the coating process the samples were polished with a polishing sheet and 6 gm diamond emulsion, then cleaned through ultrasonic cleaning in tetrachloroethylene and isopropanol. Despite the ultra- sonic cleaning diamond grains sometimes stuck in the aluminium surface, so that in future a grinding process with very fine grinding material is to be preferred to polishing.

The aluminium samples were coated in an arc-PVD plant of the type Interatom PVD 20". The arc process

500 E. Lugscheider et al./Sud'ace and Coatings Technology 74 75 (1995) 497-502

is successfully applied industrially especially for coating of tools [8]. For the coating of aluminium the known coating parameters have to be examined and eventually adapted. Within the scope of this work, in addition to the standard parameters bias, reactive gas pressure and evaporator power, the variables substrate cleaning, sur- face activation, stand-off distance, arrangement of the samples in the vacuum chamber and coating structure were investigated. The main results will be described and discussed.

For the characterisation of the wear properties of the coated aluminium samples a sand blasting test was used. The blasting parameters were chosen allowing a distinct differentiation between coated and uncoated aluminium. Various blasting materials (brown and white corundum, glass pearls) with different grain sizes were tested. The wear experiments were finally conducted with glass pearls with a grain size fraction of 92 101 pro, a jet pressure of 1 bar, an injection nozzle with 6 mm diameter and a stand-off distance of 50 ram. The blasting angle was determined to be 20 ~ in order to simulate mainly abrasive wear. Larger angles cause the effect that the amount of abrasive wear is reduced in favour of plastic deformation.

3. Results and discussion

The substrate pre-treatment is of special importance for the coating of aluminium, since a dense oxide layer forms on the aluminium surface within a very short time, which continues to grow. In order to keep this oxide layer as thin as possible and to obtain good coating adhesion, the aluminium samples should be mirror-polished just prior to the arc-PVD coating. Often

there is a metal ion etching before the actual arc-PVD coating process, in which the substrate is exposed to a bombardment of high-energy metal ions. The metal ion etching mainly improves the coating adhesion [9]. The energy of the metal ions emitted by the arc evaporator is determined through the substrate bias. If the substrate bias is too high, the energy of the particles is so high that the temperature rises too rapidly. If the bias is too low, a soft Ti coating is deposited. For the aluminium samples a short but intensive etching, which was stopped when the substrate temperature increased too rapidly, was found to be optimal. A Ti intermediate layer did not show any improvement of the coating properties.

In Fig. 4 the hardness values of substrate and coating material of coated aluminium samples vs. bias voltage are shown. With increasing voltage the coating rapidly reaches the values known from literature; however, the substrate hardness values decrease drastically. This loss of hardness is due to the higher temperature load, and it accompanies a loss of support of the substrate material. In Fig. 5 two scratches on aluminium samples coated using different bias are shown. It is also evident that the gain in coating hardness deposited with higher bias yields no wear protection due to the damaged substrate material.

The temperature load of the substrates can be reduced, in addition to cooling, through a higher sample load of the chamber and through a larger stand-off distance between substrate and evaporator. The increase in vacuum chamber filling was realised through special sample holders with a defined surface. The reduction in deposition rate due to the larger distance between samples and evaporator (Fig. 6) could be compensated through a longer coating duration. Both activities

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subs t r a t e m a t e r i a h A 1 M g S i l c o a t i n g s y s t e m : TiN reac t ive gas: N, -0. c o a t i n g PR = 2 Pa Iv = 50 A ~ s u b s t r a t e

I I I I I I

0 10 2 0 3 0 40 50

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100

80

- 6 0

0 40

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.,-

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E ¢-~

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2

Fig. 4. Hardness of aluminium substrates and arc-evaporated TiN coatings influenced by different substrate potentials.

E. Lugscheider et aL /Surface and Coatings Technology 74-75 (1995) 497-502 501

(a)

(b)

Fig. 5. Scanning electron micrographs of two scratches on coatings deposited with different bias voltages: (a) IOV; (b ) -50V.

together led to an improvement of coating properties (hardness, critical load) on the aluminium substrates.

For the blasting wear test the aluminium samples were coated without bias for 2 h with a stand-offdistance of 25 cm after a short metal ion etching (600 V bias). The results of the blasting wear test are shown in Fig. 7. Fig. 7(b) shows an uncoated sample after 30 s blasting time. In comparison to the polished surface (Fig. 7(a)) there are distinct wear marks detectable. Fig. 7(c) and (d) show coated samples after 2 and 4 min blasting time respectively. After 2 rain there is no visible indication of damage to the substrate surface due to the abrasive grain. After 4 rain the blasting material has worn off the coating at several locations and dug holes into the soft substrate. Where the coating still exists, the substrate is not attacked. The damaged coating areas can be explained, for example, by the falling out of droplets. Basically, it can be stated that a TiN hardcoating on substrates of the alloy A1MgSil reduces wear.

This investigation shows initial promising results in the arc-PVD coating of aluminium. Through further optimisation of process parameters and further develop- ments in systems technology the results may be further improved. The use of a bipolarly pulsed bias source seems especially promising for the coating of aluminium at low temperatures with the arc-PVD process. First experiments where the bias voltage was unipolarly pulsed, yielded significantly improved coating prop- erties at very low deposition temperatures. For the future, coatings are scheduled, where pulse rate, pulse intensity and pulse fi'equency are to be investigated systematically.

10

additionally coated area [cm-']

8 - ~ ~ • 900

E -m- 225

~ 6~.= AleMg~il ~ ~ ~ , , , , ~ ~ 1 ~ ~ substrate" A1MgSil o 4 . A1MgS_il ~ ~ i~ coating-sy

reactive gas: N: ~ "n - , , , , . -~ 2 PR = 2 Pa

lv = 50 A U~ = -10 V

0 n J n 1 5 20 25 3 0 3 5

source to substrate distance [cm]

Fig. 6. Deposition rates for different source-to-substrate distances and different filling ratios of the coating chamber.

502 E. Lugscheider et al./Surjitce and Coatings Technology 74 75 ( 19951 497 502

(a) (b/

(c) (d)

Fig. 7. SEM images of uncoated and coated aluminium surfaces before and after a sandblasting test. (a) Uncoated, polished aluminium surface; (b} uncoated aluminium surface after 30 s of blasting with glass beads; (c) coated aluminium surface after 2 min of blasting with glass beads (only one fracture); (d) coated aluminium surface after 4 min of blasting with glass beads.

References

[11 O. Knotek, Werkst(~'kunde llI, Vorlesungsumdruck an der RWTH, Aachen, 1983.

[2] W. Hufnagel, Aluminium-Taschenbuch, Aluminium-Verlag, Dtisseldorf, 1988.

[3] P. Brenner and H. Kostron, Z. Metallkd., 31 (19391 89 97. [4] German Industry Standard DIN 1747, Beuth Verlag, Berlin,

1983.

[5] A. Paun, Thin Solid Films, 97 (1982) 79-89. [6] S. Schiffer, Thin Solid Films, 72 (1980) 351 359. [7] J.P. Coad, Vacuum, 31 (1981) 365-370. [8] W.A. Brainard, effect of nitrogen-containing plasma adherence,

friction and wear of radiofrequency-sputtered titaniumcarbide coatings, NASA Technical Paper 1377, 1979.

[9] O. Knotek, F. L6ffler and G. Kr~imer, Surf Coat. Technol., 54/55 (19921 241-248.

[10] O. Knotek, F. L6fller and G. Kr~imer, Sur[[ Coat. Technol., 54/55 (1992) 476 481.