Surface and Coatings Technology, 62 (1993) 443—447
PVD coatings for textile machine components Th. Gries Department of Textile Technology, University of Technology, Aachen, Eilfschornsteinstr. 18, 5100 Aachen (Germany)
Abstract In the past, the development of physical vapour deposition (PVD) coatings has been concentrated on the area of cutting materials for the metal machining industry. These coatings have not yet been applied in the textile industry, where oxide ceramic materials are predominantly used for thread-contacting machine elements. In comparison with conventional materials for thread-contacting machine elements, the PYD coating has considerable advantages. Thus, investigations have shown that PVD-coated surfaces can be better adapted to the processes than can conventional materials. At the moment, coating abrasion or coating failure are hurdles which still have to be overcome before PVD coatings can be used in textile engineering on a broad basis. The service life potential of the hard material coating is not exhausted. Further investigations have to be conducted in this area.
1. Introduction In the past, the development of physical vapour deposition (PVD) coatings has been concentrated on the area of cutting materials for the metal machining industry. These coatings have not yet been applied in the textile industry, where oxide ceramic materials are predominantly used for thread-contacting machine elements . On account of its significance as one of the most important capital goods branches , and on account of the large number of wearing parts (approximately 1 million pieces per plant), a technically and economically interesting application possibility is presented in the form of the textile mechanical engineering industry, What makes this area of application technically interesting is the abrasive wearing conditions and the high demands made on the composite system, i.e. on the coating—base material. To develop suitable machine elements, it is necessary to know the existing problems involved in the use of PVD coatings. The aim of this paper is to discuss these problems. Because of the large numbers of pieces used, the base materials which have been used up until now—such as the expensive hard alloys—can be rejected on cost grounds. Therefore, new suitable base materials have to be selected on the basis of iron and light alloys. The aim of the development of PYD coatings for textile machine elements must be a low cost, abrasion-resistant PVD coating—base metal system, which is suitable for largescale production. Thus, the knowledge gained from this area is also of importance for other areas of the capital goods and consumer goods industries.
2. Demands made on thread-contacting machine elements Thread-contacting machine elements can be subdivided into three groups: active, passive and other textile machine elements. Requirement profiles for each of these groups are discussed below. The task of passive thread-contacting machine elements is to hold the thread in a process-specific position. Characteristic of these machine elements is their variety in form, which is explained both through the process and through ergonomy (Fig. I). Passive machine elements should not unnecessarily increase the thread tension during the process, so that productivity is not decreased. The function of active thread-contacting machine elements is to transfer defined forces or moments to the Thread low coefficient of friction
iow mechanical yarn damage
low thermal yarn damage
___________________________~ form diversity
~ high wear
Fig. 1. Demands on passive thread-contacting surfaces,
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PVD coatings for textile machine components
thread. To minimize the scattering of the process behavjour between the operation points, and owing to the high rotational speeds which frequently prevail, active machine elements are required to have closely lying finishing tolerances (Fig. 2). Depending either on the process used or on the required product properties, a certain precisely defined friction coefficient is required. One factor which plays a major role in the selection and improvement of both passive and active textile machine elements is an adjusted surface topography. Surface optimization gains great economic importance. Through surface optimization, improvements can be made in process safety and process speed—for example, in the case of man-made fibre production—or in quality . It must be noted that the motto “very smooth is very good” does not reign in the world of textile technology. Depending on the fibre material, taking due consideration of the yarn structure and paying due attention to the process technological requirements, it is much more the case that specially adapted technical surfaces are necessary. In addition to a surface structure which is adapted to the friction behaviour of the yarns, threadcontacting machine elements must have a surface structure which does not damage the thread mechanically. Furthermore, man-made fibres react strongly if there is an excessively high temperature in the contact zone between the thread and the machine element. In contrast, in the case of man-made fibres, threadcontacting machine elements are strongly subjected to abrasion by delustring agents, such as Ti02, which is added to them. In addition, in the case of natural fibres, this is promoted by accompanying fine dust (SiO2). Processing agents, such as coning oils or spinning lubricants, can transform themselves tribochemically into hard particles and then have an abrasion-amplifying effect. The necessary level of high air humidity in the production workshops of the textile industry also leads to corrosion problems in the textile machines. The active and passive machine elements together
represent the largest share of thread-contacting machine elements . For this reason, the remaining textile machine elements are dealt with in the group “other thread-contacting machine elements”. The demands for resistance to corrosion and abrasion are also made on this group. However, the requirements made on the surface and the geometry of these components are strongly dependent on the area where they are to be used and, therefore, cannot be put into any type of system. Figure 3, for example, shows blades used for cutting threads and a nozzle. Nozzles are used to deposit fluid resources, to structure, to strengthen and to transport threads. Through the widely advanced automation of machines and the rationalization of operational systems, multiplepoint operation dominates in the textile industry. One worker operates many machine positions—for example, over 4000 spindle points in the spinning mill. Thus, it is not possible to carry out constant monitoring of individual-operation material. For this reason, very high demands are made on the quality and on the reliability of the working stock.
3. PVD coating process Generally speaking, a material is evaporated in a vacuum in all PVD processes. This material condenses on the substrate and grows epitaxially as a coating. The differences in the processes are mainly due to the type of evaporation used. In the case of magnetron sputtering, the target is atomized by the occurrence of fast ions. In the case of Arc ion coating, a low voltage, arc discharge evaporates the target and, in the case of electron beam ion coating, the material is evaporated with an electron beam. Furthermore, by setting up a voltage on the component, it is possible to accelerate the ions present
Thread constant coefficient of friction
low mechanlcai yarn damage
iow thermal yarn damage
narrow finishing tolerances
simple surface modification
General Conditions Fig. 2. Demands on active thread-contacting surfaces.
high wear resistance
Fig. 3. Demands on other thread-contacting surfaces (examples): blades—good cutting results, wear resistance and corrosion resistance; nozzles—narrow finishing tolerance, wear resistant, low dirt accumulation, corrosion resistant.
PVD coatings for textile machine components
on the component. In this way, the epitaxially growing coating becomes stronger . When selecting a suitable coating process, attention must be paid to the surface quality required. Furthermore, a suitable coating process must have a high coating precipitation rate on account of the high number of pieces. For these reasons, magnetron sputtering was chosen for the coating. Magnetron sputtering (Fig. 4) can be used to coat nearly any base alloy, owing to its low process temperatures. In addition, the precipitated coatings form an exact copy of the topography of the base material, so that the component can be given a defined surface structure before the coating process; thus, it is not necessary to carry out the expensive finishing of the coated areas. The deposited coating offers protection against abrasion caused by the passing thread. It can transfer defined forces, owing to the structure, and offers protection from corrosion,
Advantages of the PVD.Coatlng for TextIle Machine Components
~impie surface modification
minimum use ot 1 Lexpensive hard msteriaiaj 1~ variety of
kind to the thread
/ / / / ~
Fig. 5. Advantages of PVD coatings for textile machine components.
could be reduced at high rotational speeds (up to 120 000 rev mm 1 and more) in the field of textile technology. A change in the surface caused by deposits is avoided by having a low porosity. Furthermore, the low coating temperature enables warp-sensitive textile
4. Advantages of PVD coating for thread-contacting machine elements
machinery parts to be coated. The example of the texturing disc is taken to illustrate clearly these advantages . Further practical experience
Owing to the great importance of surface design in textile technical machine elements, PVD coatings have a major advantage for this application over other hard material surfaces. This advantage stems from their topographic copying property, because the other hard material surfaces usually have to be finished using expensive processes (Fig. 5) Through the use of the PVD coating in textile technology, the surface modifications which are required for different products could be produced using cost-effective methods. It was proven that it was no longer necessary to finish the coating (see below), A further advantage is that the basic body can be optimized using lightweight construction methods. In this way, bearing stressing and fluctuation tendencies
reports are available on weaving and spinning machine elements, as well as on cutting .
5. Coating trials using the example of the texturing disc Man-made fibres are extruded in a smooth condition and have to be structured when they are used in areas such as apparel, sport clothing and automobile textiles. One of the most important methods for structuring is false-twist texturing (Fig. 6). In this method, the thread is temporarily twisted via the twister’s friction discs and the twist is heat set in a few milliseconds. The thread is wound up in a twist-free state. However, the individual endless fibres have a permanent helix-like twist structure
Fig. 4. Coating process.
Fig. 6. False-twist texturing machine for structuring man-made fibres.
PVD coatings for textile machine components
and are thus given properties, such as thread bulk, heat insulation capacity and elasticity. Up until now, two groups of friction disc materials have been used for texturing. The hard material discs
(A1203 sintered ceramic, nickel diamond dispersion coat-
ings material account and ofdetonation-moulded thediscs. resistance abrasion. coatings) However, are used they on cause soft process-disrupting a greater degree abrasion. This of damage isto particularly The to the polyurethane thread noticeable than soft do ina material discs, which are gentle on the thread, have considerably shorter service lifetime. Thus, a hard material disc is sought which is as kind to the thread as the polyurethane disc. The importance of the surface structure during the transfer of the friction moment onto the thread is made clear in Fig. 7. For this reason, in this initial trial of PYD coatings on friction discs, the main emphasis was placed on adapting the surface topography. First, the friction discs made of heat duroplastic wrought aluminium alloy were precision turned and sand blasted. Several of these discs were then heat burred or ground with different reaction times. These discs were sputtered with a titanium-aluminium nitride ((TiAl)N) coating. In the texturing experiments, these PVD-coated aluminium discs were compared with the standard disc materials, i.e. A1203 sintered ceramic and polyurethane. The number of twists introduced to the thread per minute was selected in this case as a measure for evaluating the process properties (Fig. 8). More than 1 3000 necessary twists m’ for or more than 1.8change million in twists min are the desired structure. Although all the discs had an arithmetic average peakto-valley height which lay in the range 0.2—0.5 J.tm, the required structural change effect in the man-made fibre was obtained from the conventional friction disc materials, and then only from the sand-blasted aluminium and the sand-blasted and heat-burred aluminium,
.~ .~ 0 ‘~
A1203 PU t thb tg s shb sg Fig. 8. Twists produced in the texturing zone: yarn—polyester PES 167 dtex f32; friction discs—Ø = 52 mm, d = 9 mm, seven working discs; conventional materials—PU, polyurethane; A1203, sintered ceramic; PYD coatings—t, turned; thb, turned and heat burred; tg, turned and ground; s, sand-blasted, shb, sand-blasted and heat burred; sg, sand-blasted and ground.
In regard to fibre friction, the sand-blasted and PVDcoated disc outdid the A12O3 disc and almost reached the values of the polyurethane disc (Fig. 9). This result confirms that PVD-coated textile machine elements can be designed in a more thread-friendly way than can conventional hard materials. In the case of the PVD coating on these texturing discs, the first signs that the coating was faulty appeared within one week. The coating flakes at certain points under dynamic stress (Fig. 10). The failure of a hard and brittle coating on a more ductile base material is known in textile engineering as “mirror breakage” or the “eggshell effect”. The tendency for this coating failure mechanism is increased by a low coating thickness of 2 jim, by the selected base material and by the coating material. Also, the base material involved is a heat-precipitated aluminium alloy which loses half of the initial hardness of 100 HB, despite the low coating temperature. The coating material (TiA1)N was not developed for textile machine elements but rather for metal-cutting machining 49
____ ~ 50 40
~ E 0
0.2 0.05 ~
0 Al 203 PU Fig. 7. Thread path of a man-made filament yarn on a friction disc.
Fig. 9. Amount of yarn wear. Details as Fig. 8.
PVD coatings for textile machine components
_____ ________ ________ _____
Fig. 10. Wear of the PVD coating on friction discs: base material, AlSiMgl; surface finish, sand-blasted; coating material, (TiAI)N; coating thickness, 2 ~tm.Original magnification 30 x.
still have to be overcome before PVD coatings can be used in textile engineering on a broad basis, The service life potential of the hard material coating is not exhausted. In the broad spectrum of possible harder base materials, limits are set on account of the high numbers of workpieces. This means that hard alloys do not come into question as base materials. The complex compound between base material and coating materials, and the constantly necessary adaption of the surface structure require further intensive research and trials of this laminate system. New coating systems have to be developed, more suitable base materials investigated and process parameters optimized accordingly. It is only when service lifetimes of between one and five years are reached that textile technical advantages of PVD coatings can be used in practice.
with high temperature peaks. Further investigations have to be conducted in this area.
6. Summarizing remarks and outlook
I Tb. Gries, Strukturkeramik II, TASK, Aachen, 1992, p. 140. 2 Statistisches Handbuch für den Maschinenbau, VDMA Frankfurt, 1991.
In comparison with conventional materials for threadcontacting machine elements, PVD coating has considerable advantages. Thus investigations have shown that PYD-coated surfaces can be better adapted to the processes than can conventional materials. At the moment, coating abrasion or coating failure are hurdles which
3 B. Wuiffiorst and Tb. Gries, Die Bedeutung von Oberflãchen für das Reibungsverhalten mit Garnen, Final Rep. AIF/Gesamttextil 8154, Aachen, 1993. 4 B. Wulfhorst, Th. Gries, 0. Knotek and B. Bosserhoff, IngenieurWerkstoffe, 7/8 (1993) in press. .. .. . 5 R. Haefer, Oberfiachen- und Dunnschicht-Technologle, Part I, Springer, Berlin, 1987. 6 Th. Gries, DWI-Rep. 111 (1993) 5. 533.