Method of coating metallic and ceramic substrates

The invention is concerned with a reduction of the surface roughness of titanium-aluminum-nitride hard material layers produced by the combined cathodic arc discharge vaporization process and the imbalanced magnetron coating process (ABS method) or by the pure cathodic arc discharge vaporization process.

FIELD OF THE INVENTION 
The invention relates to a method of coating metallic and ceramic 
substrates such as cutting tools or forming tools and the like with TiAlN. 
DESCRIPTION OF PRIOR ART 
It is known that titanium aluminum nitride can be used as an alternative to 
high wear resistant TiN hard material layers. The improved oxidation 
resistance relative to TiN in particular makes this coating material a 
likely candidate for coolant-free chip-forming machining in automatic 
production lines, for example in the modern motorcar industry. In 
addition, titanium-aluminum-nitride hard material layers show an elevated 
resistance to abrasive wear in the presence of carbides, which comes to 
expression in a particularly advantageous manner in the machining of grey 
cast iron and cold working steels. 
Titanium aluminum nitride is nowadays manufactured using all known PVD hard 
material coating processes, namely cathode sputtering, cathodic arc 
discharge vaporization, and the low voltage electron beam process. 
Moreover, combination processes are known such as the combination of the 
cathodic arc discharge vaporization process as an etching source in the 
PVD process with the imbalanced magnetron as a coating tool, or the 
combination of either the cathodic arc discharge vaporization process with 
the low voltage electron beam vaporization process or with the magnetron 
during coating. 
A process of this kind is known from European patent specification 0 439 
561 for the coating of substrates which is also termed an ABS process in 
which the layer to be applied is manufactured by the incidence on the 
relevant substrate of condensing particles of the plasma produced by means 
of a gas discharge, with both an arc discharge vaporization process and 
also a cathode sputtering process being effected. It is important that the 
arc discharge vaporization process is only carried out during a first 
coating phase and the cathode sputtering process during a second coating 
phase following this first phase. The substrate is bombarded during the 
arc discharge vaporization process with ions, and the energy and ion 
current density are selected with this arrangement in such a way that the 
substrate surface is cleaned and partially removed by ion etching. 
With this process the ion energy and the ion current density are preferably 
selected for the bombardment of the substrates with ions, which takes 
place during the arc discharge vaporization process so that an anchoring 
of the layer to be built up in the substrate material is brought about. 
In the apparatus for carrying out this method there are provided, in a 
vacuum chamber filled with a process gas, a substrate holder which is 
electrically insulated relative to the vacuum chamber, a cathode which is 
occupied by the coating material and is likewise electrically insulated 
relative to the vacuum chamber, and an anode which is required for the arc 
discharge. The cathode is selectively connectable to an arc discharge 
supply or to a cathode sputtering supply and can be connected to negative 
potentials such that an arc discharge takes place in a first current 
circuit between the cathode and the anode or such that a cathode 
sputtering discharge takes place in a second circuit. 
An apparatus for the coating of substrates using an arc discharge process 
and a cathode sputtering process is known from EP 0 459 137 Al which has 
an evacuatable container in which at least one target, an associated 
anode, and also a substrate carrier are arranged, with the associated 
current and voltage supplies being provided and with a magnet arrangement 
being associated with each target. The magnet arrangement, consisting of a 
center pole permanent magnet and oppositely poled permanent edge magnets 
surrounding and spaced from the latter, is displaceably mounted relative 
to the associated target in such a way that in the target area a magnetic 
field which enables cathode sputtering is generated in a first position 
adjacent to the target and a magnetic field which enables an arc discharge 
is generated in a second position spaced from the target. 
OBJECT OF THE INVENTION 
The present invention is concerned with PVD methods (physical vapor 
deposition methods) which preferably use either only the cathodic arc 
discharge vaporization process as a process source or use the latter in 
combination with cathode sputtering methods (magnetron, imbalanced 
magnetron).

DESCRIPTION OF THE INVENTION 
The starting point for the invention is the observation that during the 
metal ion etching process of the substrate, so-called "droplets" are 
deposited on the substrate surface which during subsequent coating prove 
to be the cause for local growth defects and which contribute to an 
enormous increase of the surface roughness of the 
titanium-aluminum-nitride layer. 
This applies both for the case that the PVD coating is continued after the 
metal ion etching with a method which does not itself generate any 
"droplets", such as for example with an imbalanced magnetron, or also with 
a pure cathodic arc discharge vaporization process in which an additional 
"droplet" formation can be observed during the coating and thus a further 
increase of the surface roughness takes place. 
As can be shown with respect to the behavior of TiAl and Ti as the target 
material in the cathodic arc discharge process, the number of "droplets" 
which arise during the metal ion etching is dependent on the melting 
temperature of the target. Higher melting temperatures lead to reduced 
"droplet" formation. Accordingly, the concentration of defects with a TiN 
coating (melting point of Ti:1670.degree. C.) is substantially lower than 
with the TiAlN coating (melting point for 50:50 TiAl:1450.degree. C.). 
Two possibilities have been shown for reducing these effects. If one 
replaces the metal ion etching step using TiAl by an argon etching 
process, then the development of growth defects is almost completely 
suppressed. 
However, it has also been shown that the bond strength of TiAlN layers is 
greatly impaired if one replaces the metal ion etching step by an etching 
step with inert argon ions. 
In order to understand the influence of the target material during the 
cathodic arc discharge vaporization process with respect to the creation 
of "droplets" and to the bondability of TiAlN layers, the metals Al, TiAl, 
Ti, Zr, Cr, Nb, Mo were investigated as a metal ion etching vapor. During 
these etching experiments, the substrate material of steel was placed as a 
negative bias of -1200 volts. During the cathodic arc discharge process an 
argon pressure of 10.sup.-3 mbar prevailed. The distance between the test 
substrate and the arc discharge source amounted to ca. 40 cm on average. 
With an increasing melting temperature the diameter of the "droplets" 
reduced from a maximum of 15 .mu.m for Al to approximately a maximum of 
1.5 .mu.m for Nb. 
Parallel to this the "droplet" concentration of 8.times.10.sup.4 "droplets" 
per cm.sup.2 for Al reduced to 1.times.10.sup.4 "droplets" per cm.sup.2 
for Nb. 
The roughness of the steel substrate following the etching also reduced 
substantially from R.sub.a =0.044 .mu.m for Al to Ra=0.005 .mu. for Nb. 
The named metals cover a melting point range from ca. 660.degree. C. for Al 
to 2470.degree. C. for Nb. 
All materials of TiAl (1450.degree. C.), Ti (1670.degree. C.), Zr 
(1850.degree. C.) to Cr (1870.degree. C.) follow this tendency with 
respect to an inclination to "droplet" formation. 
A particularly low level of the "droplet" formation is only achievable in 
the case of chromium and molybdenum. 
Although Cr has a melting point close to that of zirconium, the surface 
roughness already had the same low value as for the metal niobium as a 
cathodic arc discharge source, namely R.sub.a =0.005 .mu.m. The high vapor 
pressure of the chromium even far below the melting point can be regarded 
as the explanation. Even following the solidification, the diameter of the 
"droplets" at the substrate evidently reduces in a notable manner. 
As an explanation for this circumstance, FIGS. 1a to 1g show a series of 
surface profiles. 
The peak height is proportional to the diameter of the "droplet" and 
corresponds for all materials, apart from Al, to a maximum of 4 .mu.m. In 
this respect it should be noted that this is a factor 2 smaller than for 
aluminum. Here the maximum value is approximately 8 .mu.m. 
Parallel to this it was found that the bond strength of TiAlN layers 
deposited with the imbalanced magnetron on high speed steel HSS with TiAl, 
Cr or Ti in the metal ion etching process is comparably high in practice, 
namely critical load 50-70 N, whereas for Zr and Nb this characteristic 
value sank to 20-40 N. 
If one compares the roughness of 3 to 4 .mu.m thick TiAlN layers on high 
speed steel HSS etched with Cr ions with layers of this kind which were 
etched with TiAl, then one finds a value of R.sub.a =0.05 .mu.m for the Cr 
etched samples and a value of R.sub.a =0.15 .mu.m for the TiAl etched 
samples. 
On the basis of these positive results spiral drills with a diameter of 8 
mm which have been coated with TiAlN to ca. 3 .mu.m thickness using the 
ABS process were investigated on flat polished test surfaces. The ABS 
method is a combined process of metal ion etching in a cathodic arc 
discharge and of coating using the imbalanced magnetron. 
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The following cutting parameters were used: 
Material GG25 (grey cast iron) 
cutting speed 60 m/min 
feed 0.3 mm/rpm 
depth (blind bore) 28 mm 
lubricant dry 
The following cutting results were obtained: 
uncoated 20 holes 
coated*/TiAl etching step 
100 holes 
coated*/Cr etching step 
220 holes. 
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The composition of the layer amounted to 42 atomic % Ti and 58 atomic % Al 
at 100 atomic % N. 
All experiments were carried out in a four-target coating plant of the type 
HTC-1000/4 ABS. 
FIG. 2 shows a schematic cross-section through a plant HTV1000/4 ABS 
operating in accordance with the ABS method. Four vertically arranged 
cathodes (target surface: 600.times.200 mm.sup.2) can be used either in 
accordance with the cathodic arc discharge vaporization principle or as an 
imbalanced magnetron. 
All four cathodes can be magnetically coupled together with the aid of 
electromagnets (concentric coils), so that a magnetically closed space 
arises in which the substrate carriers are located which rotate and are 
additionally arranged on a rotary plate. The transition from the cathodic 
arc discharge to the imbalanced magnetron can be adjusted via the position 
of the permanent magnets relative to the target surface. During the 
experiments that were carried out, three cathodes were occupied with TiAl 
(50:50) targets and one cathode with a Cr target. 
It was possible to further improve the cited cutting results when the Cr 
target used in a four-target PVD plant for the etching step was also used 
during the TiAlN coating. In this case, the drilling result could be 
increased to 380 holes. Similar positive values, namely 420 holes, were 
obtained when one added carbon-containing gases such as CH.sub.4 or 
C.sub.2 H.sub.2 etc. to the process gases Ar and N when using the Cr 
target during the TiAlN coating with the imbalanced magnetron. In this 
respect the carbon-containing gas was only added to the already present 
gas mixture during the last phase of the coating, for example after the 
expiration of 75% of the total coating time. 
Finally, comparative drilling tests were also carried out with drills which 
had been coated with ABS or in a purely cathodic arc discharge deposition 
technique. 
Here the following result was shown: 
ABS with TiAl in the metal ion etching stage and pure cathodic arc 
discharge vaporization coating showed similar results in the 
above-described test, that is to say the drilling result lay at ca. 100 
holes. 
As a result, proof has been submitted that, with regard to the dry cutting, 
the greatest care must be developed with respect to the layer roughness. 
Rough layers are clearly exposed to an increased frictional resistance. As 
a result elevated cutting temperatures arise which in turn lead to 
shortened lifetimes. The results presented show clearly that the 
phenomenon of "droplet" formation can be decisively influenced on the 
basis of a suitable choice of material. 
In the following scheme the process sequence for the ABS coating with Cr or 
TiAl as metal ion etching variants is illustrated. 
Both methods lead to very different results with regard to the roughness of 
the surface layers. Cr etched substrates are substantially smoother than 
TiAl etched substrates. 
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SMOOTH SURFACE ROUGH SURFACE 
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