DEVELOPMENT OF SURFACE TREATMENT FOR THERMALLY AND MECHANICALLY LOADED PARTS OF COMBUSTION ENGINES

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1 DEVELOPMENT OF SURFACE TREATMENT FOR THERMALLY AND MECHANICALLY LOADED PARTS OF COMBUSTION ENGINES Pavel NOVÁK 1, Jiří VYSTRČIL 1, Ondřej CHOCHOLATÝ 2, Alena MICHALCOVÁ 1, Dalibor VOJTĚCH 1 1 Ústav kovových materiálů a korozního inženýrství, Vysoká škola chemicko-technologická v Praze, Technická 5, Praha 6, panovak@vscht.cz 2 BUZULUK a.s., Buzulucká 108, Komárov, Ondrej.Chocholaty@buzuluk.cz Abstract Several parts of combustion engines (such as piston rings or exhaust valves) are loaded by a combination of thermal cycling, adhesive-abrasive wear and corrosion by combustion gases. The materials selection for these conditions is quite complicated and in some cases the coatings (e.g. electrolytic chromium coatings or TiN layers prepared by PVD) are applied. In this work, the applicability of Ni-ceramics composite coatings and Fe-Al intermetallic layers was tested. Preparation technologies of these layers were developed for steel and cast iron substrates. Microstructure, mechanical and tribological properties and thermal stability of the layers were determined. Keywords: combustion engine; surface treatment; microstructure; wear; thermal stability 1. INTRODUCTION Moving parts of the combustion engines, as engine valves and piston rings, suffer from a mixture of abrasive and adhesive wear, thermal cycling and corrosion attack by combustion gasses. In the case of engine valves, erosion can be also observed. For these applications, steel and cast iron are usually applied. For current high-compression high-power engines, the properties of these materials are insufficient. Therefore, several types of surface treatment are applied. For the above mentioned parts, nitriding, hard chromium or PVD coatings are currently available and industrially applied [1]. For highly demanding applications, the development both basic material and surface treatment are in progress. Iron and titanium aluminides were successfully tested as the material of engine valves without a surface treatment [2], [3], [4].Other way is the development of the innovative surface treatment. In this field, composite coatings play important role [5]. In this work, two different approaches in surface treatment are presented. First one is the use of nickel-matrix composite coatings reinforced by ceramic particles. Second type of surface treatment uses high wear and oxidation resistance of iron aluminides by using aluminide layers. 2. EXPERIMENTAL As the basic material, heat-treated ductile cast iron was applied. The composition of the material is given in Tab. 1. Before surface treatment, the material was degreased in commercial detergent, pickled in a solution of HCl in water (1:1) with the addition of hexamethylentetramine and rinsed with distilled water. Nickel-matrix composite coatings were prepared by electroless plating in the bath containing nickel sulphate, sodium hypophosphate, ammonium chloride and ammonium citrate. Ammonia was applied for the ph adjustment to the value between 8 and 10. As the reinforcement, Al 2 O 3 and SiC particles were added to the bath after 10 min of coating process. The bath was stirred to achieve uniform particles distribution. The composite layers were heat-treated by annealing at 400 C for 1h.

2 Iron aluminide layers were produced by pack aluminizing at 600 C for 4 h using aluminium powder (99.99 % purity) and activator. Tab.1 Chemical composition of the substrate [wt. %] C Si Mn P S Cr Ni+Cu Fe <0.3 < 0.05 < 0.2 <2.0 bal. Microstructure of prepared materials was observed by an Olympus PME3 light microscope and a TESCAN VEGA 3 LMU scanning electron microscope equipped with OXFORD Instruments INCA 350 EDS analyser (SEM-EDS). The abrasive wear resistance was evaluated by using a modification of the pin-on-disc method, where pin was the tested material and disc was a P1200 grinding paper. The applied load was 5.8 N and the sliding distance was defined as 2.5 km. The wear rate was calculated from the measured weight losses by the equation (1) [6]: m 1000 w, (1) l where w, Δm, ρ and l are wear rate [mm 3 m -1 N -1 ], weight loss [g], density [g.cm -3 ] and sliding distance on the grinding paper [m], respectively. Thermal cycling test was applied in order to reveal the susceptibility of the coatings to the spallation due to thermally-induced stresses. For this test, the temperature of 350 C and total duration of 408 h were selected. The temperature choice is based on the temperature in the car engine. Cycle length was 24 h. 3. RESULTS AND DISCUSSION The microstructure of the substrate is displayed in Fig.1. It consists of rounded graphite particles in bainitic matrix. Fig.1 Microstructure of the ductile iron used as the substrate

3 Composite coatings produced by electroless nickel plating with ceramic particles are presented in Fig. 2. Coating reinforced by corundum particles with the size of 0.5 to 5 µm is very uniform around the whole sample, reaching the thickness of approx. 19 mm, see Fig.2a and Tab.2. Below the composite layer, pure Ni- P sub-layer can be observed. This one was produced by 10 min nickel plating before the addition of Al 2 O 3 particles. Fig.2 Microstructure of the composite coatings: a) Ni-Al 2 O 3, b) Ni-SiC On the other hand, the thickness of Ni-SiC composite layer is very variable (Fig.2b, Tab.2). The reason of this fact lies probably in the size of SiC particles. The particle size reaches up to 20 µm on the contrary to extremely fine Al 2 O 3. In the case of coarser particles, the erosive effect on the coating during deposition is highly probable, thus causing inhomogeneity in layer thickness. In addition, large particles complicate further accommodation of the nickel coating. The coating reinforced by fine corundum particles reaches also slightly higher hardness than in Ni-SiC layer (Tab.2). The content of nickel in both types of coatings is approx. 13 wt. %. Due to high phosphorus content the nickel coating is amorphous. To increase the hardness, wear resistance and thermal stability of the coating, the material was heat treated (400 C, 1h). The hardness of the coating increased to approx. two times higher values, see Tab.2. Tab.2 Thickness, hardness and chemical composition of studied layers thickness [µm] hardness [HV0.01] hardness after heat treatment [HV0.01] content of P or Al [wt.%] Ni-Al 2 O 3 19 ± ± ± (P) Ni-SiC 16 ± ± ± (P) Fe-Al 58 ± ± (Al) Second tested surface treatment method was the application of aluminide layers, prepared by pack aluminizing. The structure of this layer is formed by Fe 2 Al 5 iron aluminide (Fig.3). The presence of this phase was confirmed by the chemical microanalysis by SEM-EDS, which determined the aluminium content of

4 approx. 55 wt. %, corresponding to Fe 2 Al 5, in the whole cross-section of the layer (Tab. 2). In this layer, micropores can be observed. This layer is characterized by high hardness (nearly 1300 HV), similar to heattreated nickel-phosphorus coatings. However, the hardness is achieved in one step; no further heat treatment is necessary. Fig.3 Microstructure of Fe-Al layer Abrasive wear resistance tests proved that all surface treatment methods improve the wear resistance of the ductile cast iron. The most significant decrease of the wear rate (i.e. increase of the wear resistance) was determined in the case of Ni-SiC composite coating (Fig.4). Nickel-corundum composite coating and aluminide layer provide similar values of the wear rate. It shows that the abrasive wear rate is low when the layer is supported by larger hard particles. On the other hand, hard but more brittle layer of aluminides does not improve the abrasive wear behaviour so significantly. However, in the car engine the parts as exhaust valves and piston rings are loaded by the sliding wear (a combination of adhesion and abrasion) in lubricated conditions and thermal shocks rather than the strongly abrasive wear as during this simplified test. In that case, the wear rate can be different. The cyclic thermal loading test revealed no problems with adherence of all layers. With the composite coatings as well as aluminide layer, neither delamination nor measurable oxidation was observed. Therefore all tested layers can be recommended for the use in thermal cycling to 350 C as in the car engine.

5 Fig.4 Wear rate of studied coatings (P1200 grinding paper, distance 2500 m, normal load 5.8 N) Fig.5 The weight of the surface-treated samples during the thermal cycling test 4. CONCLUSION In this work, the Ni-ceramics composite coatings and Fe-Al intermetallic layers were tested as the surface treatment of materials for mechanically and thermally loaded parts of combustion engines. Composite coatings were prepared by autocatalytic electroless nickel plating with the addition of Al 2 O 3 or SiC particles, while the iron aluminide layer was obtained by pack aluminizing. Preliminary tests of abrasive wear resistance and resistance to thermal cycling revealed that these layers can be potentially applicable to the

6 conditions in the car engine. To prove the applicability, wear tests that simulate conditions in car engine have to be applied and mechanical properties at elevated temperatures should be determined. ACKNOWLEDGEMENT This research was financially supported by Czech Science Foundation, project No. P108/12/G043. REFERENCES [1] E.P. BECKER, Trends in tribological materials and engine technology, Tribology International, 37 (2004) [2] E.A. LORIA, Quo vadis gamma titanium aluminide, Intermetallics, 9 (2001) [3] S.C. DEEVI, V.K. SIKKA, Nickel and iron aluminides: an overview on properties, processing, and applications, Intermetallics, 4 (1996) [4] A. CHOUDHURY, M. BLUM, Economical production of titanium-aluminide automotive valves using cold wall induction melting and centrifugal casting in a permanent mold, Vacuum, 47 (1996) [5] A. POSMYK, Influence of material properties on the wear of composite coatings, Wear, 254 (2003) [6] P. NOVÁK, D. VOJTĚCH, J. ŠERÁK, Wear and corrosion resistance of a plasma-nitrided PM tool steel alloyed with niobium, Surface and Coatings Technology, 200 (2006)