Structure-phase transformation in electrochemical boron containing coatings by thermal treatment

Transcription

1 published at the WTK 2005, Chemnitz, D, September 29 th - 30 th 2005 Structure-phase transformation in electrochemical boron containing coatings by thermal treatment Vasyl Pokhmurskii, Roman Mardarevych Karpenko Physico-Mechanical Institute of NASU, Lviv, Ukraine pokhmurs@ipm.lviv.ua Bernhard Wielage, Hanna Pokhmurska, Andreas Wank Institute of Composite Materials, Chemnitz University of Technology, Chemnitz, Germany Abstract Combined method of obtaining two-layer nickel-boron-chromium coatings is developed by successive electro deposition of nickel-boron composite and chromium layers and thermal treatment in inert environment at C. Elaborated chemical composition of bath, parameters of electrolysis and hydrodynamic regimes of suspension mixing provide an electro deposition of high quality nickel-boron layer showing uniform distribution of boron micro powder particles with 5-6 wt.-% content. Influence of annealing parameters on microstructure, distribution of elements and phase content of double layer coatings are studied. Wear and fatigue tests of low alloyed steel specimens with deposited composite coatings indicate direct dependence of these properties on the coating microstructure. Wear resistance of coatings with coarse grained structure formed after annealing at C is times higher than that of coatings with small grained structure obtained at 850 C. Fatigue tests show reverse dependence. Fatigue limit of steel specimens with small grained structure coatings is 30% higher than for coarse grained structure. 1 Introduction Surface strengthening using nickel based composite electrochemical coatings (CEC) is one of the ways for effective improvement in durability and reliability of machine parts and equipment. In these coatings hard refractory particles (carbides, borides, nitrides) or solid lubricants (graphite, calcium fluoride, molybdenum sulphide) are mainly used as dispersed phases [1]. Composite structure of coatings combines high hardness, wear resistance and chemical inertness of dispersed particles with plasticity and heat conductivity of metal matrix and thus excellently satisfies the requirements on tribotechnical materials. Appreciable improvement of CEC properties is achieved with the help of thermal treatment. Annealing of coatings with particles that are inert in relation to a matrix is accompanied by certain structural changes of the matrix, i.e. recrystallization, internal stress relaxation, increase of cohesion between particles and matrix and elimination of micro defects. Thermal treatment of coatings, which contain particles of elements that are chemically active with respect to the matrix, is even more effective. Annealing of such coatings is accompanied by partial or full dissolution of particles in the matrix due to diffusion interaction with formation of solid solutions or chemical compound phases. Thereby the amount of stress concentrators is reduced and coating durability in terms of wear resistance increases. For instance, nickel-boron coatings obtained by co-deposition of galvanic nickel and dispersed amorphous boron particles after heat

2 treatment show considerably improved tribological properties [2]. This is due to formation of a new heterogeneous microstructure in the nickel-boron CEC, which consists of matrix (hard nickel solid solution) and nickel borides Ni 3 B. After thermal treatment sufficient volume content of boride grains with high hardness, i.e GPa is observed. Thus sliding wear resistance increases in comparison to initial coatings (without heat treatment) by 6-8 times, in comparison to hard chromium by times and to diffusive boride coatings on low alloyed steel by times [3]. Alloying of nickel coatings by chromium has positive influence on mechanical and protective properties. In this context the combined method of producing two-layer coatings by successive electro deposition of nickel-boron and chromium layers followed by heat treatment in inert atmosphere is developed [4,5]. The influence of heat treatment parameters on structural transformations in coatings and on some properties in use is studied. 2 Experimental schedules Coatings are produced on low alloyed steel substrates by consecutive deposition of composite electroplated µm thick nickel-boron and µm thick galvanic chromium coating. Suspension for composite electroplated nickel-boron coating is prepared on the base of sulphide-chloride electrolyte for nickel plating. Powder of amorphous boron is preliminarily cleaned and dispersed by an ultrasonic device to ensure dimensions of powder particle agglomerates less than 5 µm. The developed electrolyte composition, electroplating parameters and mixing conditions allow to obtain high quality composite coatings with 5-6 wt.-% boron. Annealing of coatings is carried out in inert atmosphere for 1-5 h at C. Structure, phase and chemical composition of coatings are characterized by means of metallographical methods, scanning electron microscopy (SEM) and X-Ray diffraction (XRD) analyses using Cu K α irradiation. Temperatures of phase transformations are determined by means of differential thermal analysis (DTA) in dynamic regime in helium atmosphere with heating rate 10 C/min. Sliding wear tests are performed in block-on-ring configuration without lubrication under normal pressure MPa and sliding velocity 0.67 m s -1. Fatigue behavior of coated specimens is investigated under pure bending load. 3 Results and discussion 3.1 Thermodynamics of boride phase formation Annealing of two-layer coatings causes diffusive interaction of components of both layers, redistribution of elements, formation of new phases and structures. With the purpose to predict the resulting phase composition of coatings a thermodynamical estimation of probability of solid phase reactions of borides formation in the system Ni-Cr-B using tabular data and charts of calculation is carried out [6]. The negative value of Gibb s energy for reaction products is the general condition of reaction development towards stable equilibrium (fig. 1). Results of calculation of free energy change for boride phases in the temperature interval of K show that formation of borides CrB and CrB 2 is thermodynamically most probable (- G= and kj/mol respectively). The formation potentials for borides Ni 3 B and Ni 2 B are more positive (- G = kj/mol). So probability of such reactions is smaller.

3 Fig. 1: Change of free energy of boride phases formation in system Ni-Cr-B [6] Thus annealing of two-layer coating that contains boron in nickel matrix must be accompanied by the diffusion of boron atoms into chromium layers as there is driving force due to chemical potential gradient of boron between these coating layers. Kinetic factor (change in mobility of atoms as a result of composition change, structure and residual stress state) as well as thermodynamics factor of diffusion process will limit reactivity of the system. 3.2 Phase transformations Coating component interaction with formation of boride phases takes place mainly as reactionary diffusion with appropriate values of thermal reaction effects. Temperature intervals of solid phase reactions are determined by differential thermal analysis for three coating compositions. Phase transformations do not take place in two-layer nickel-chromium coatings at temperature up to 950 С (fig. 2). Heating of nickel-boron CEC is accompanied by phase transformations indicated by exothermic peaks with different intensity at temperatures 258 C, 487 C and 629 С (fig. 2). a b c Fig. 2: Heating thermograms of coatings; а) Ni-Cr; b) CEC Ni:B; c) CEC Ni:B-Cr Exothermal effect at 258 С is minor and is not detected by simple thermocouple and is not represented on the heating curve. Probably it can be related to insignificant

4 oxidization of boron caused by the presence of impurities in inert gas. Presence of other, more intensive peaks on DTA curves is caused by solid phase reactions resulting in nickel boride formation which is confirmed by XRD analyses (table 1). Table 1: Phase composition of CEC nickel-boron and nickel-boron-chromium coatings after heat treatment according to XRD analyses temperature [ С] coating nickel-boron nickel-boron-chromium 20 Ni Ni, Cr 260 Ni Ni, Cr 420 Ni, (Ni 3 B) 435 Ni, Cr, (Ni 3 B) 490 Ni, Ni 3 B Ni, Cr, Ni 3 B, (CrB) phases of small content are marked with handles Exothermal effect is maximal for the nickel-boron-chromium CEC. Powder samples are overheated more than 200 С (fig. 2). Such considerable heat emission is caused by additional solid state interaction of chromium and boron at the interface of layers and formation of chromium borides (table 1). 3.3 Microstructure of CEC Thus, the electrodeposited nickel-boron and nickel-boron-chromium coatings have increased reaction ability at relatively low, i.e С annealing temperatures. This is caused by considerable concentration of all known types of crystal lattice defects in galvanic coatings, i.e. vacancies and structural defects which serve as paths for accelerated diffusion. Homogeneous dispersion of boron particles and absence of oxide films on their surfaces as well as presence of significant amount of atomic and molecular hydrogen in nickel and chrome layers (0.1 and 0.45 wt.-% respectively) decrease activation energy of solid state reactions in CEC [7]. Hydrogen desorption at all stages of heating is accompanied by an increase of defect density and as a result diffusion process rates increase. Irregular shape of boron particles observed in initial structure (fig. 3) gradually changes to spherical shape during annealing (fig. 4). Annealing of coatings up to temperatures of 850 С causes significant structural changes. Increase of annealing temperature up to 950 С results in reduced dimensions of the undissolved (non reacted) boron particles. a b c Fig. 3: Microstructure of Ni:B-Cr coating before heat treatment; Ni:B / Cr interface (left), Ni:B / substrate interface (middle), Ni:B layer (right)

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6 850 C, 2h 950 C, 2h 1,050 C, 1h Fig. 4: Microstructure of Ni:B-Cr coatings after heat treatment; general view (left), microstructure of upper layer (middle), microstructure of interface between coating and substrate (right) Two hours exposure at 950 C provides strong dissolution of boron particles. In optical micrographs double phase structure of nickel borides distributed in nickel matrix is clearly visible. Increase of annealing temperature up to 1050 С results in full consolidation of boride grains which are distributed in alloyed nickel matrix (fig. 4). In addition to transformations in Ni:B layers annealing causes distribution of elements and diffusive interaction of both coating layers and Ni:B layer with the steel substrate. Boron, as the most active coating component, has dominating role in diffusion processes. Line scan X-ray microanalyses prove boron diffusion into the whole thickness of the chromium layer. With increase of temperature and annealing duration boron concentration increases in this layer. Thus, annealing of two-layer coatings boron diffusion results in borating of the chromium layer with creation of chromium boride on the coating surface. A number of factors, i.e. thickness of initial layers, boron content in Ni:B layer and annealing

7 temperature-time function, determine phase composition and structure of the resultant surface state.

8 3.4 Properties of annealed coatings Microhardness Microhardness of boride grains in the surface layer is GPa and that of the chromium matrix is 2-3 GPa. On the layer interface a transitional zone of Ni-Cr solid solution is created. The lower layer has two-phase composition structure, i.e. Ni 3 B dispersed in nickel matrix which at the interface with the steel substrate transforms into a zone of Ni-Fe solid solution with dispersed inclusions of iron borides. Microhardness of boride grains in the Ni:B layer is GPa, and that of the nickel matrix is GPa. For increasing annealing temperature in the range C the thickness of transitional diffusion zone expands from 2-3 to 6-8 µm. Wear resistance Tribological tests show that composite coatings outperfom galvanic hard chromium and even more hardened steel (HRC 40-45) concerning sliding wear resistance (fig. 5). Steel-steel pairs work in the mode of oxidation wear for loads exceeding 1 MPa. Increase of load up to 2 MPa causes intensive destruction of the oxide film, appearance of plastic deformation zones and adhesive wear which is accompanied by drastically increased wear rate. 1 - uncoated steel substrate 2 - galvanic chromium 3 - CEC Ni:B (850 С, 1 h) 4 - CEC Ni:B (1000 С, 2 h) 5 - CEC Ni:B-Cr (850 С, 1 h) 6 - CEC Ni:B-Cr (1000 С, 2 h) Fig. 5: Results of sliding wear tests with hardened steel counterbody, duration: 8 h At 2 MPa load galvanic chromium is also worn out intensively, but because of high hardness and heat resistance it is not worn by adhesive wear. Under dry friction conditions the composite coatings show the lowest friction coefficient ( ), which indicates high carrying ability of boride structures and positive role of triboreaction oxide films acting as a lubricant. Boride structures formed by annealing at 850 C through insufficient volumetric contents of boride grains in the metal matrix show lower wear resistance compared to two-layer coating annealed at 1,000 C. For annealing at 1000 C coatings contain more than 60% of boride grains and an accordingly small content of metal matrix. Fatigue strength Influence of coating on steel durability under cyclic loading conditions is an important factor to evaluate coating potential. In most cases of hard wear resistant coating use for surface protection negative influence on steel durability is observed. Galvanic

9 nickel and chrome coatings decrease fatigue limit of low alloyed steel by about 30% and 50% respectively. Ni:B composite coatings without thermal treatment show the same effect (fig. 6). Such decrease of steel fatigue limit is caused by a number of factors, i.e. hydrogen pickup by coating and steel in the coating deposition process, unfavorable distribution of residual stresses and defects of galvanic and composite coatings that act as strain concentrators. 1 - uncoated steel 2 - galvanic nickel 3 - Ni:B CEC, not heat-treated 4 - galvanic chromium Fig. 6: Fatigue limit of low alloyed steel with coatings 1 - uncoated steel 2 - N:B CEC (850 С, 1 h) 3 - Ni:B CEC (950 С, 2 h С, 1 h) 4 - Ni:B-Cr CEC (850 С, 1 h) 5 - Ni:B-Cr CEC (950 С, 2 h С, 1 h) The results of fatigue tests for coated specimens after thermal treatment confirm the positive influence of annealing on strength characteristics (fig. 6). After thermal treatment at 850 C the Ni:B coating does not change the fatigue limit and two-layer Ni:B-Cr coatings increase the limit up to 10%. This can be attributed to the presence of dispersed boride grains that are coherently connected with the matrix lattice in the structure which can retard fatigue progression. After thermal treatment at 950 C the coatings decrease the strength of steel insignificantly due to increasing brittleness. 4 Conclusions Two-layer composite coatings are developed applying a combined method of twolayer nickel-boron-chromium coating deposition. Microstructure, elemental distribution and phase content of double layer coatings after thermal treatment in inert environment at C are investigated. Optimisation of heat treatment parameters provides formation of surface layers with uniform distributed boride grains that supply high wear resistance and strength of steel parts. An increase of wear resistance by factor is observed for coarse grained coating structure which forms at high annealing temperatures, i.e C. A small grained structure formed at 850 C annealing temperature improves fatigue limit of low alloyed steel by 30% compared to the coarse grained structure.

10 Literature [1] Saifullin, R.S.: Non organic composite materials, Moscow, Chemia, 1983 (in Russian) [2] Epik, A.P., Yu.A. Guslienko, N.N. Sverdlik: Obtaining and some properties of composite boron coatings, Protective coatings on metals (1983) 4, pp (in Russian) [3] Koskov, V.D., V.P. Permiakov, N.N. Nogtiev: Kinetics of formation and properties of complex alloyed boron coating on iron, Publishing house: Metallurgy of Ferrous Metals, (1983) 6, pp (in Russian) [4] Pokhmurskii, V., R. Mardarevich: Many layer composite coatings of nickel-boronchromium system, Physical chemical mechanics of materials (1998) 4, pp (in Ukrainian) [5] Mardarevich, R., V. Pokhmurskii: Corrosion resistance of galvanic boron containing coatings in sulphur acid solution, Physical chemical mechanics of materials, (2002) 3, pp (in Ukrainian) [6] Gurvich, L.V., I.V. Veic, V.A. Medvedev: Thermodynamic properties of individual substances, Moscow, Publishing House Science, 1982 (in Russian) [7] Povetkin, V.V., I.M. Kovenskii: Structure of electrolytical coatings, Moscow, Publishing House Metallurgy, 1983 (in Russian)