Surface Engineering of Metals - Principles, Equipment and Technologies Part 20 potx

– Physico-chemical: by removal through detachment of the more stable contaminants in washing baths (e.g., chemical solvents) and alkaline aque- ous solutions, assisted by ultrasonic vibration. Not in all deposition techniques and not all components are cleaned by all techniques, as enumerated in the above sequence. In some techniques it is only possible to brush off dust and grease and to degrease chemically (usu- ally in specially prepared baths and organic solvents). Degreasing in differ- ent baths is usually separated by single or double rinsing and a possible drying cycle. Quite often, the final stage is ultrasonic washing in trichloroet- hylene. Ultrasonic washing in freon baths has been eliminated, mainly on account of the detrimental effect of freon on the earth’s atmosphere because freon causes depletion of the ozone layer. Table 6.10 shows an example of the operation of cleaning twist drills of 100 mm diameter, practically used in the Institute for Terotechnology in Radom, Poland [127]. Table 6.10 Initial cleaning of twist drills, prior to coating deposition (From Bujak, J., et al. [127]. With permission.) 6.4.4.3 Final (ion) cleaning Final cleaning is carried out in the vapour depositor’s chamber, directly prior to deposition, and it has the following aims [127-129]: – precise cleaning of component surface, – activation of component surface, – heating of component to desired temperature. Final heating is accomplished by ion beam techniques and is termed ion etching or ion beam etching. In practice, the process of ion cleaning, fol- Type of operation Washing medium Time[min.] Temperature [∫C] Degreasing 1) 10g/dm 3 NaOH 10g/dm 3 Na 3 PO 4 •10H 2 O 10g/dm 3 Na 2 CO 3 5g/dm 3 wetting agent or 2) 20g/dm 3 NaOH 20g/dm 3 Na 3 PO 4 •10H 2 O 0.03 g/dm 3 oleic acid, oxyethylized by seven particles of ethyl oxide 31 5065 Rinsing I distilled water 1.66 (100 s) 80-95 Rinsing II distilled water 1.66 (100 s) 20 Drying pure air 10 110 Rinsing III trichloroethylene 6 80 Rinsing IV trichloroethylene + ultrasonic 520 Rinsing V trichloroethylene vapours 10 90 Drying air 5 110 © 1999 by CRC Press LLC lowed by coating deposition, may be carried out without any interruption between the two operations during which residual gas atoms could con- dense on the substrate. This is possible because at low pressures employed in ion beam techniques of deposition, a monolayer of atoms condenses on a substrate that is not bombarded by ions within seconds [13]. During ion etching some structural components of the vapour depositor are also sputtered, especially load fixtures. For this reason, a condition of effective ion beam etching is limiting to a minimum vapour deposition on the substrate of structural materials (not coating material, e.g., titanium), and ensuring that the rate of sputtering is greater than the rate of condensation of residual gases. Meeting the second condition enables heating of the substrate (by ion bombardment or by additional heaters) which causes a rise of des- orption of residual gas atoms adsorbed by the surface, as well as of ions of the working gas used to clean the surfaces which are trapped in the substrate [13]. A rise of substrate temperature causes an increase of the force of adhe- sion of coating to substrate. Overheating the substrate may, however, cause a loss of acquired mechanical properties of substrate, e.g., exceeding of tem- pering temperature causes a loss of hardness. For this reason, in the case of metallic loads, heating is carried out at a temperature approximately 50 K lower than that of tempering (150 to 350ºC). In ion beam heating, tem- perature depends on shape, mass and size of load surface [127]. Fig. 6.21 Schematic diagrams of systems for ion etching techniques: a) with load as cathode; b) in metallic plasma magnetron; c) in metallic plasma, assisted by arc dis- charge; d) in triode system with hollow cathode; e) in triode system with arc source; f) by ion beam. © 1999 by CRC Press LLC Fig. 6.22 Rate of load heating in different techniques of ion etching: 1 - traditional sputtering with load as cathode; 2 - in triode system with hollow cathode; 3 - in metallic plasma from magnetron discharge; 4 - in metallic plasma from arc discharge. (From Bujak, J., et al. [127]. With permission.) Fig. 6.23 Rate of deposition/sputtering in metallic plasma from arc discharge vs. po- tential: a) dependence on material of sputtered cathode; b) dependence on distance between load and cathode. (Fig. a - curve Zr and Ti (1) - from Andreev, A.A., et al. [130]. With permission; Curve Al and Ti (2) - from Vyskocil, J., and Musil, J. [131]. With permission from Elsevier Science; Fig. b - from [14]. With permission.) Four techniques of ion beam etching are known and in use, of which two are utilized regularly while the remaining two rather seldom [128]. Ion etching with the load as cathode - cathodic etching. This technique, the oldest, simplest and most widely used, may be carried out in the majority of vapor depositors equipped with systems for substrate polarization (Fig. 6.21a) and with systems for rapid extinguishing arc microdischarges. Its disadvan- tages are low rate of etching (Fig. 6.22), low rate of heating and possibility of occurrence of arc microdischarges, on account of high voltage applied to the load [127]. Ion etching in metallic plasma. This technique consists of utilization of magnetron discharge (Fig. 6.21b) or/and arc discharge (Fig. 6.21c), with low target sputtering currents and with a negative bias of the load. The density of metallic plasma bombarding the load is independent of its size which allows © 1999 by CRC Press LLC precise control of sputtering current and energy of heavy ions of target metal (by a change of polarization voltage) and, hence, a change of rate of load heating (Fig. 6.22). A disadvantage of this technique is the possibility of oc- currence of both substrate sputtering, as well as deposition of a metal layer on it (Fig. 6.23). This may happen when a boundary voltage is exceeded, which for a titanium target at 10 -3 Pa pressure is approximately 700 V. For each type of metallic plasma and for a given pressure there exists a bound- ary etching voltage [129-131]. Ion etching in a triode system with hollow cathode. In this technique the cathode is a vacuum chamber, usually grounded, while the anode is the vacuum feed-through, connected to the positive pole of the high volt- age generator. This feed-through takes up approximately 0.1% of total cath- ode surface. The triode system is constituted by: the load, the anode and the plasme source, e.g., hollow cathode (Fig. 6.21d) or arc source (Arc Enhanced Glow Discharge - AEGD method; Fig. 6.21e). This technique allows obtaining of significantly greater etching rates than by conventional cathode etching techniques. It is possible to freely select voltage, hence, etching current. High values of ion beam current at low voltages on the load form favorable condi- tions for uniform etching of loads with complex geometry, including those with small diameter cavities and holes. These advantages mean that the tech- nique lends itself particularly well to etching (and heating) of small preci- sion tools, dielectric materials and residues from various chemical compounds on the load surface [127]. Ion beam etching. This technique allows thorough cleaning of loads made from any material (Fig. 6.21f) and simultaneous modification of both the substrate, by shallow implantation, as well as of the coating [130]. Its disadvan- tages are complex equipment and accompanying systems [127]. 6.5 Service characteristics of coatings deposited by PVD techniques 6.5.1 General PVD techniques are characterized by the following features: – Possibility of application of raw materials in the form of pure metals and gases, in place of often harmful compounds. – Broad possibilities of choice of coating materials, thus broad range of properties of deposited coatings. – Relatively high deposition productivity with utilization of special- ized vacuum vapor depositors. – Quite high deposition costs (high investment costs), although more than compensated by a severalfold increase in life of coated objects or by possibil- ity of their achieving properties impossible to achieve by other techniques. – Necessity of maintaining high degree of cleanliness and strict adher- ence to procedures which call for high operator qualifications. © 1999 by CRC Press LLC – Ecological friendliness of deposition processes (no harmful products of chemical reactions and need for their disposal). Well-deposited coatings have one feature in common, i.e., in principle, they simultaneously embrace all the basic service characteristics, such as very good tribological and anti-corrosion properties, coupled with high deco- rative properties. Some coating materials additionally feature unique optical and dielectric properties. It is essential to emphasize two special characteristics: - the same coating materials deposited by different techniques usually do not feature the same coating characteristics; - properties of coatings deposited by one technique, with the utilization of the same materials, do not have to be the same because their chemical compo- sition may be different; they may form stoichiometric, substoichiometric and superstoichiometric compounds. For example, coatings of titanium ni- tride with general chemical formula of TiN x may contain nitrogen within the range of 28 to 50% (atomic) which corresponds to the value of 0.42 ≤ x ≤ 1 [13]. The chemical composition of the layer depends, of course, on the conditions of deposition, in stricter terms, on the proportion of reagents in the plane of the substrate, in accordance with the formulas: Ti + N 2 = TiN + 0.5N 2 (6.1) 2Ti + N 2 = 2TiN (6.2) 3Ti + N 2 = Ti 2 N + TiN (6.3) 4Ti + N 2 = Ti 2 N + TiN + Ti (6.4) This proportion depends on the rate of condensation of titanium on the substrate and on partial pressure of nitrogen (uniform in different zones of the working chamber), as well as on the degree of ionization which, similarly to degree of condensation, is not uniform in all zones of the working chamber [13]. Usually, the thickness of coatings does not exceed several microns (most often 2 to 5 µm) for monolayer coatings and 15 µm for multi-layer coatings [121]. In some cases, it may reach 100 µm, as in CrN coatings. The rate of deposition varies and in most cases it is within the range 3 to 18 µm/h. Table 6.11 shows properties of some coatings deposited by the reactive magnetron sputtering tech- nique. As opposed to CVD techniques, in the development of PVD techniques a striving is observed for departure from low-temperature processes and for reaching into the higher temperature range, up to a substrate tem- perature of 600ϒC, in an effort to activate the reaction of compound for- mation and to enhance adhesion of coating to substrate through a partial diffusion-type connection. In the future, new modifications of techniques © 1999 by CRC Press LLC Table 6.12 Colors of thin, hard coatings (From different sources. ) Fig. 6.24 Colors of coatings of Ti(C,N) titanium compounds vs. content of carbon and nitrogen. (From Knotek, O., et al. [134]. With permission.) By utilizing compounds of titanium with carbon (carbides) and with ni- trogen (nitrides), it is possible to obtain a quadruple spectrum of colors, gently blending one into another (Fig. 6.24). 6.5.3 Tribological properties 6.5.3.1 Coating of tooling and machine parts Good tribological properties of PVD coatings (Table 6.13), coupled with low thermal conductivity (see Tables 6.5 to 6.7) and resistance to temperature effects (see Table 6.11), are the basic factors which caused that these coatings Coating Color Hardness [HV0.015] TiN yellow-golden (from bright yellow, through red-yellow to bright brown) to gray-silver 2300-2700 ZrN pale golden 2300-3200 HfN yellow-green 2700-3100 Ti(C x N 1-x ) (for x = 0.05 −50) reddish-golden-brown 2450-2900 Zr (C x N1-x) (for x = 0.05 −20) golden 3250-3450 Zr (C x N 1-x ) (for x > 0.9) silver 3300-3600 (Ti x Al 1-x )N (for x = 0.1 −70) golden-brown-black 2400-2900 (Ti x Zr 1-x )N (for x = 2 0−80) golden 2400-2900 Cr(C,N) silver 1500-2000 TaN yellow-silver metallic 1700-2100 SiC from gray to yellow 2800-3200 © 1999 by CRC Press LLC tool application within 7 to 9 months, assuming continuous one-shift-per- day operation [121]. Different types of tooling are often coated by layers of: TiN, TiC, WC, Cr 3 C 2 , CrN, Ti(C,N), (Ti,Al)N, (Ti,Zr)N, TiC/TiN, TiC/TiB 2 and TiC/Ti(C,N)/ TiN [94]. Performance of such tools, coated with an anti-wear layer, depends pri- marily on coating material, type of tool and its material, material being machined, as well as type and conditions of the machining operation. Since all these values vary within certain limits, their appropriate selection can and should be carried out empirically, based on results of laboratory test- ing. It should be, however, taken into consideration that the results of such tests are usually better than those of industrial applications [14]. 6.5.3.2 Tool performance Effect of material on tool life. A lower coefficient of friction of the coat- ing on steel, conducive to lesser heat being dissipated in the zone of con- tact of tool with machined material, hence a lowering of cutting edge temperature, and the fulfillment by the layer of the role of a heat barrier, cause a reduction of cutting forces (Fig. 6.25), improve tool working con- ditions and result in an extension of tool life, in comparison with that of uncoated tools (Fig. 6.26) [135-145]. Fig. 6.25 Dependence of drilling conditions in steel drilling by NWKa 10 mm dia. twist drills on type of coating: a) cutting moment; b) axial force. (From Smolik, J., et al. [138]. With permission.) Fig. 6.26 Comparison of results of drilling by twist drills of 6 mm diameter, made of high speed steel, uncoated and coated by TiN and (Ti,Al)N. Drilling depth: 15 mm. Drilling speed: 20 m/min, feed rate: 0.2 mm/rev. Drilled material: steel (hardness: 243 HRC). (From Münz, W.D. [132]. With permission.) © 1999 by CRC Press LLC Fig. 6.27 Mean increase of tool life of tools made from high speed steel, coated by anti-wear coating: a) thread taps; b) thread milling cutters; c,d) cutting tools; e) reamers; f) forming tools: inserts, punches, dies, sintered carbide pins. (From Smolik, J., et al. [138]. With permission.) © 1999 by CRC Press LLC Fig. 6.28 Life of various tools coated by TiN. (From [145]. With permission.) Fig. 6.29 Increase in life of tools coated by different types of anti-wear coatings vs. cutting speed: a) twist drills; b) end mills; c) cutters. (Fig. a - from Smolik, J., et al. [138]; Fig. b and c - from König, W., and Koch, K.F. [146]. With permission.) Life extension of different types of tooling. The average life of tool- ing with hard coatings is 2 to 5 times longer than that of uncoated tools. In particular cases, life extension may even exceed 10 times and more. For tools used in threading operations in industrial conditions this extension is 2-fold; in lathe operations, reaming and extrusion it may be 4 to 5 fold (Fig. 6.27) [138], while in stamping operations it is even higher (Fig. 6.28). Effect of type of treatment and its parameters on tool life. In all types of cutting operations there is an effect of parameters on the life of tools (Fig. 6.29). For each type of tool and each type of coating there is an optimum intensity of treatment (cutting speed, depth, feed rate), usually greater than the intensity of treatment by an uncoated tool, different for © 1999 by CRC Press LLC Fig. 6.30 Effect of cutting depth during reaming on life of reamers with Ti(C,N) coat- ing. (From Smolik, J., et al. [138]. With permission.) two different types of tools with same type of coating and different for same tools with different types of coating. Generally, a rise in treatment intensity is conducive to a shortening of tool life [147], even with coatings (Fig. 6.30) [138]. 6.5.4 Anti-corrosion properties Coatings which are resistant to corrosion should be tight and sufficiently thick. They should not exhibit a columnar structure. Usually, the thickness of anti-corrosion coatings exceeds 10 µm [13]. Hard coatings, provided they are very tight and appropriately thick, are usually chemically resistant. This is especially the case with carbides and nitrides which exhibit very good chemical resistance in the lower temperature range and only strongly concentrated acids cause their slow dissolution [92]. A coating which is often used for corrosion protection is titanium ni- tride (best if the ratio of TiN to Ti 2 N is 1:1) which at room temperature is chemically inactive and features small chemical affinity to materials with which it mates in service. It is resistant to acids and alkalis, it reacts weakly with concentrated acids and strong oxidizing agents and the more amorphous its structure, the more resistance it exhibits. Oxidation of coat- ings in dry air begins near the lower limit of glow temperatures, i.e., approxi- mately 650ºC, causing a gradual change off coloring from golden to brown after cooling [148]. The (Ti,Al)N coating appears to have even better anti- corrosion properties than TiN. The titanium nitride as a material is neutral with respect to the human organism, featuring total corrosion resistance in human saliva and does not exhibit any cytotoxic effects in body fluids. Thanks to these properties it is used for coating working surfaces of surgical and dentistry instru- ments, as well as of dentures. The thickness of coatings utilized in medi- cine is small - usually about 1 µm [149]. © 1999 by CRC Press LLC [...]... utilization of ion etching (in Polish) Seminar on Selected Problems of Surface Engineering, Rzeszów-Bystre (Poland), 1992, p.32 Andreev, A.A., Gavrilko, I.V., Kunchenko, V.V., and Sopyrkin, L.I.: Investigation of some properties of Ti-N2, Zr-N 2 condensates, obtained by deposition of plasma streams in vacuum (KIB technique) (in Russian) Fizika i Khimia Obrabotki Materialov (Physics and Chemistry of Material... 6 4-6 8 Vyskocil, J., and Musil, J.: Arc evaporation of hard coatings: Process and film properties Surface and Coatings Technology, No 4 3-4 4, 1990, pp 29 9-3 04 Müntz, W.D.: Continuous hard coating Metal Progress, No 8, 1987, pp 6 5-6 8 Randhawa, H., and Johnson, Ph.C.: New developments in decorative vacuum coatings Metal Finishing, Sept 1988, pp 1 9-2 2 Knotek, O., Prengel, H.G., and Brand, J.: Deposition of. .. structure and properties of thin films Proc.: 1st National Autumn School, Szczyrk (Poland), Oct 1979 111 Wolin, Z.M.: Ion-plasma techniques of obtaining wear-resistant coatings (in Russian), Technologia Lyogkikh Splavov (Technology of Light Alloys), No 10, 1984, pp 5 5-5 8 112 Handbook of thin film technology, Ed L.I Massel, R Glang, McGraw-Hill Book Co., New York 1982 113 Deposition technologies for films and. .. 1987, pp 10 9-1 24 118 Dieter, G.E.: Mechanical metallurgy McGraw-Hill Book Co., New York 1976 119 Posti, E., and Nieminen, I.: Influence of coating thickness on the life of TiNcoated high speed steel cutting tools Wear, 129, 1989, pp 27 3-2 83 120 Movchan, V.A., and Demchishin, A.V.: Investigation of structure and properties of thick vacuum condensates of nickel, titanium, tungsten, aluminium oxide and zirconium... Crystallization of multi-phase layers from pulsed plasma (in Polish) Warsaw University of Technology, Warsaw 1987 125 Messier, R., Giri, A.P., and Roy, R.A.: Revised structure zone model for thin film physical structure Journal of Vacuum Science and Technology, A2(2), 1984, pp 50 0-5 11 126 Burakowski, T.: Status quo and development trends of surface engineering (in Polish) Part III: Classification and general... machining tools and tools for cutting non -metals (such as laminations of plastics and wood), aluminum and non-ferrous metals to extend their life Furthermore, such coatings are used on optical lenses in order to increase their chemical, mechanical and thermal resistance, on microchips, as insulation and mechanical pro- © 1999 by CRC Press LLC changes of illumination intensity Optical properties of glasses,... of formation of thin metallic layers (in Polish) PWN, Warsaw 1974, pp 3 3-3 9 11 Kienel, G.: PVD-Verfahren und ihre Anwendung zur Herstellung verschleisshemmender Schichten Proc.: 37 Härterei-Kolloquium, Wiesbaden 1981 12 Burakowski, T.: Methods of manufacture of superficial layers - metal surface engineering (in Polish) Proc.: Conference on Methods of Manufacture of Superficial Layers, Rzeszów (Poland),... Service properties of TiN, TiC and Ti(C,N) coatings (in Polish) Proc.: II Polish Conference on Surface Treatments Czestochowa-Kule, 1993, pp 15 9-1 63 Miernik, K., Walkowicz, J., and Smolik, J.: Deposition of AlN layers by collimation magnetron sputtering Surface and Coatings Technology, No 98, 1988, pp 129 8-1 303 Walkowicz, J., Smolik, J., Miernik, K., and Bujak, J.: Anit-wear properties of Ti(C,N) layers... vacuum arc method Surface and Coatings Technology, No 81, 1996, pp 20 1-2 08 Smolik, J.: Interface role in forming anti-wear properties of multilayer TiC/ Ti(C,N)/TiN coating obtained by vacuum arc method Ph D thesis (in Polish) Warsaw University of Technology, Warsaw 1998 Bujak, J., Miernik, K., Smolik, J., and Walkowicz, J.: Obtaining of TiN and TiAlN layers by the magnetron and vacuum-arc methods (in... 6.31 Effect of temperature of hard coating on stainless steel on: a) resistance to oxidation (specimens were held at all temperatures for 1 h); b) hardness drop; 1 - stainless steel; 2 - TiN; 3 - Ti(C0.3N0.7); 4 - TiC; 5 - (Ti0.75Al0.25Cr0.25)N; 6 - (Ti0.5Al0.5)N (Fig a - from Münz, W.D [132].) Corrosion resistance at elevated temperatures of the majority of hard coatings is also good - decidedly better . reddish-golden-brown 245 0-2 900 Zr (C x N1-x) (for x = 0.05 20) golden 325 0-3 450 Zr (C x N 1-x ) (for x > 0.9) silver 330 0-3 600 (Ti x Al 1-x )N (for x = 0.1 −70) golden-brown-black 240 0-2 900 (Ti x Zr 1-x )N. Problems of Surface Engi- neering, Rzeszów-Bystre (Poland), 1992, p.32. 130. Andreev, A.A., Gavrilko, I.V., Kunchenko, V.V., and Sopyrkin, L.I.: Investiga- tion of some properties of Ti-N 2 , Zr-N 2 . application of raw materials in the form of pure metals and gases, in place of often harmful compounds. – Broad possibilities of choice of coating materials, thus broad range of properties of deposited