ТЕПЛОФИЗИКА ВЫСОКИХ ТЕМПЕРАТУР, 2014, том 52, № 1, с. 22-29
IMPROVEMENT OF MICROSTRUCTURAL AND MECHANICAL PROPERTIES OF PLASMA SPRAYED Mo COATINGS DEPOSITED ON Al-Si SUBSTRATES BY PRE-MIXING OF Mo WITH TiN POWDER
© 2014 D. Debasish, S. Mantry*, D. Behera, B. B. Jha
CSIR-Institute of Minerals and Materials Technology, Bhubaneswar-751013, India *E-mail: mantrysisir@gmail.com Ph : +916742379399
Abstract—The present work embodies development of a new class of mechanically improved Mo-TiN coating material using plasma spray technique. The coatings are developed on Al—Si alloy at different torch input power levels ranging from 15 kW to 30 kW. Pre-mixing of TiN with molybdenum enhances adhesion strength and hardness of the coatings. Maximum adhesion strength of 22 MPa (±0.75) and hardness of 748 HV (±30) are found for the coating when molybdenum is pre-mixed with 10% TiN. FESEM micrographs of the as-sprayed coatings showed formation of plate-like structures of splats which indicates TiN sites as reinforcement in Mo matrix. X-ray diffraction study reveals the formation of both MoO2 and TiN as minor phases in the coating microstructure. The significant enhancement of mechanical properties like adhesion strength and hardness is attributed towards the presence of these phases.
DOI: 10.7868/S0040364414010074
INTRODUCTION
Plasma spray is one of the most significant techniques employed to develop coatings of both refractory metals and ceramics. The main advantages of plasma spray technique include the formation of ceramics with fine, equi-axed grains without columnar defects at high deposition rates. It is a typical thermal spraying process that combines particle melting, quenching and splat formation in a single operation [1, 2]. To ensure better adhesion, it is a wide practice to blast the surface of the substrate with sand grit. At blasting angle of 90° and spraying angle of 90°, better adherence has been reported than at any other blasting or spraying angle [3]. The microstructure of plasma sprayed cermet coatings is found to be inhomoge-neous [4]. Within the lamellae, it has a columnar structure. The size of the splats lies in between 10—100 ^m in diameter and 1—3 ^m in thickness. R. McPherson [5] has reported that coating over the substrate has both columnar and equiaxed microstructure. The coating produced by plasma spray technique may have several applications in aerospace industry. For example, it can increase the shelf life of the turbines which are continuously exposed to high temperature environment [6, 7]. Similarly, various automobile parts like piston crown, piston ring, cylinder block and turbocharger components etc. can also be coated to protect them from wear. It is reported by F. Rastegar and A.E. Craft [8] that plasma sprayed molybdenum/chromium carbide coatings have higher wear resistances than electroplated molybdenum carbide coatings. Molybdenum has
high melting point of 2623°C and hardness of 5.5 in Mohs scale [9]. It forms MoO2 in the plasma zone which is a well known anti-friction material. MoO2 has lower coefficient of friction than that of pure molybdenum. It also has the ability to prevent expansion and softening in extreme temperature environment. Similarly, titanium nitride has a melting point of 2930°C with high hardness, low coefficient of friction, and excellent wear resistance. It protects cutting and sliding surfaces from getting damaged [10—16]. In the present work, molybdenum premixed with titanium nitride was sprayed using a plasma torch on Al-Si alloys to obtain requisite hardness and adhesion strength of the coatings with an objective to find a suitable coating for the turbine blades and casing of the turbo-chargers mostly found in diesel vehicles. Generally, the turbocharger turbine blades and casing are made of Al—Si alloy. The components get worn out due to high temperature exhaust gases as well as abrasive particles present in engine oil, thereby, do not last the specified operating life. The blades get fully damaged before the specified operating life. The objective of this investigation is to coat the blades with molybdenum premixed with titanium nitride powder to enhance its hardness and adhesion strength, so that it provides the required wear resistance at high temperature. Four different compositions of molybdenum and titanium nitride powders (100% Mo, 95% Mo + 5% TiN, 90% Mo + 10% TiN and 85% Mo + 15% TiN) were prepared and coated at four different power levels on the
substrate and characterized for their hardness, microstructure, deposition efficiency and adhesion strength.
EXPERIMENT
Titanium nitride and molybdenum powders of spray grade quality were obtained from Metallizing Equipment Co. Pvt. Ltd. Jodhpur, India. Required amount of TiN was mixed homogeneously with molybdenum to get the desired composition like 95% Mo—5% TiN, 90% Mo—10% TiN, and 85% Mo-15% TiN. Prior to spray deposition, Al—Si alloy substrates having composition Al ~ 90 wt % and Si ~ 10 wt % and dimension 100 x 60 x 5 mm3 were blasted with grits of size 60 to roughen the surface in order to provide mechanical interlocking with the particles.
The coatings were obtained at four operating power levels in the range of 15—30 kW keeping the duration of the spray constant for all cases. The process parameters involved in the experiments are given in the Table. The equipment used to produce the samples is an 80 kW plasma spray system from M/s Metallisation, UK installed at CSIR-IMMT, Bhubaneswar, India. This is a typical atmospheric plasma spray system which works in non-transferred arc mode.
RESULTS AND DISCUSSION
Deposition efficiency. Deposition efficiency is defined as the ratio of amount of powder deposited on the substrate after plasma interaction to the amount of powder material fed to the torch from the powder feeders. It depends on many factors such as melting point of powder, stand-off distance, particle size in the
Deposition efficiency, %
15 20 25 30
Torch input power, kW
Fig. 1. "Variation of deposition efficiency with torch input power level: 1 - 100% Mo, 2 - 95% Mo - 5% TiN, 3 -90% Mo - 10% TiN, 4 - 85% Mo - 15% TiN.
Process parameters for development of Mo-TiN coating during plasma spraying
Process parameter Range of operation
Current 400-650 A
Torch input power 15-30 kW
Primary gas (Argon) flow rate 40-60 L min-1
Secondary gas (Helium) flow rate 8-15 L min-1
Stand-off distance 100 mm
Feed rate of the powder 25 gm min-1
Particle size 20-60 p.m
powder etc. For the given stand-off distance and coating material, torch input power plays a vital role. As shown on Fig. 1, the deposition efficiency is found to be higher for the higher torch input power. The deposition efficiency values go up to 66% when 30 kW input torch power is used for 85% Mo-15% TiN mixture. Better deposition efficiency of Mo coating is observed with increase in TiN concentration. This may be attributed to melting of more powder and better interlocking with the substrate. Similar type of observations on conventional TiO2 powders has been reported in [17]. They have attributed increase in the deposition efficiency to higher molten degree of powders. Montavon et al. [18] reasoned that increase in gun current results in higher plasma temperatures, thus in-
Coating thickness, p.m
15 20 25 30
Torch input power, kW
Fig. 2. Variation of coating thickness with torch input power level: 1 - 100% Mo, 2 - 95% Mo + 5% TiN, 3 -90% Mo + 10% TiN, 4 - 85% Mo + 15% TiN.
Hardness, HV
1000 , 1
П 2 3
800 u-4
600
400
200
Й
&
x ■::
Г
№
15 20 25 30
Torch input power, kW
Adhesion strength, MPa 24
18
12
15 20 25 30
Torch input power, kW
Fig. 3. "Variation in Vickers hardness of the Mo—TiN coating on Al-Si substrate with torch input power: 1 - 100% Mo, 2 - 95% Mo—5% TiN, 3 - 90% Mo-10% TiN, 4 -85% Mo-15% TiN.
Fig. 4. Variation of adhesion strength of Mo-TiN coating on Al-Si substrate with torch input power: 1 - 100% Mo, 2 - 95% Mo-5% TiN, 3 - 90% Mo-10% TiN, 4 -85% Mo-15% TiN.
6
0
creases the heat transfer rates, and thereby the deposition efficiency;
Coating thickness. Figure 2 shows the thickness values for different compositions of Mo-TiN plotted versus torch input power. The thickness of the coatings on the Al-Si substrates were determined using an optical microscope (Leica DMI 3000M) at different locations on each specimen and then the mean coating thickness was calculated. The coating thickness is found to be in range of 330-370 ^m. For this purpose, the samples were mounted vertically to view under the microscope. It is observed that with increase in torch input power, the thickness of coating increases. Molybdenum premixed with 10% and 15% TiN shows higher coating thickness of 362 and 365 ^m respectively for 30 kW torch input power. The occurrence of such high coating thickness at high torch input power and high TiN concentration is also supported by the high deposition efficiency as observed previously. In other words, more amount of powder have melted fully and resulted in better inter-splat bondage.
Coating hardness. The micro hardness measurements of the specimens were carried out using Leco Micro Hardness Tester comprising of monitor and microprocessor based controller, with a load value of 0.493 N and loading time of 20 s. Several observations were taken for each sample and the average value was determined in terms ofVickers hardness (HV). For the Al-Si alloy, the hardness value obtained for various composition of coating powder with different power level is shown in Fig. 3. Maximum hardness of748 HV found in case of 90% Mo-10% TiN composite coating at 20 kW torch input power level. The hardness val-
ues for the rest of specimens vary from 600 to 750 HV. Figure 3 provides the detailed comparision of hardness values obtained for Vickers for all the coatings done. Presence of super hard phases like TiN and MoO2 is expected to contribute to the hardness of the Mo-TiN coatings [19, 20].
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