EVALUATION OF THE BOND STRENGT OF THE THERMALLY SPRAYED COATINGS

Transcription

1 EVALUATION OF THE BOND STRENGT OF THE THERMALLY SPRAYED COATINGS Michaela KAŠPAROVÁ a, František ZAHÁLKA b, Šárka HOUDKOVÁ c a ŠKODA VÝZKUM Ltd., Tylova 1/57, Plzeň, Česká republika, michaela.kasparova@skodavzykum.cz b ŠKODA VÝZKUM Ltd., Tylova 1/57, Plzeň, Česká republika, frantisek.zahalka@skodavzykum.cz c ŠKODA VÝZKUM Ltd., Tylova 1/57, Plzeň, Česká republika, sarka.houdkova@skodavzykum.cz Abstract Thermally sprayed coatings are one of the possibilities of protection of machine parts surface. It ranks among successful type of protection, which is commonly used against undesirable influences of the environment, such as different types of wear (abrasion, erosion, adhesion, cavitation,...), oxidation, high temperatures, etc. In industrial applications several technologies for the preparation of thermally sprayed coatings are used: HVOF/HVAF, arc, flame, plasma and other unique technologies. Each of these technologies is characterized by typical properties of final coatings. Coatings properties (microstructure, oxide content, phase formation, cohesion between individual splats, cracks, porosity, surface hardness, microhardness, surface roughness, wear resistance, coating bond strength, etc.) significantly differ in the dependence on the used technology. Investigation of the coating behaviour on the coating-substrate boundary and the cohesive strength between individual structure particles belongs to the evaluation of coatings mechanical properties. Coatings are bonded with the base material (substrate) mainly by mechanical interlocking. Further the van der Waals forces (molecular interaction) and diffusion of elemental species across splat boundaries participate. Further important factors which influence the coating strength are the melting and localized alloying of the contact surfaces between particles and between the substrate and adjoining particles. All these factors significantly influence the adhesive-cohesive strength of the coatings. The coating adhesive-cohesive behaviour very depends above all on two spray parameters: the flame temperature and particles kinetic energy. In the presented study, the adhesive-cohesive strength of HVOF and Flame spray thermally sprayed coatings were investigated, especially the Inconel 718 and the WC-Co/NiCrFeBSi coatings were taken into the account. Keywords: bond strength, tensile strength, adhesion, cohesion, thermally sprayed coatings 1. INTRODUCTION The tensile adhesive strength (bond strength) of the thermally sprayed coatings is evaluated using the standardized methods in accordance with EN 582 Standard (Thermal spraying Determination of tensile adhesive strength) [1] or in accordance with the ASTM C American Standard (Standard test method for the adhesion or cohesion strength of thermal spray coatings) [2]. These tests are used not only for the evaluation of the influence of the based material on the adhesive strength of the coatings material but also for the determination of the suitable surface treatment (degree of substrate roughness) of the base material before the coating spraying. The tests are further used for the quality control of the sprayed coatings. The base principle of the tests is as follows, see Fig. 1: The tested sample covered on one front side by the coating is joined with the loading and bolstering member using the suitable adhesive medium (glue). The joining has to be symmetrical and the glued joint has to be loaded by the required force. After the thermal curing of the glued joint the tension test is performed in accordance with EN The tensile strength R H is calculated as the quotient of the maximal load F m and of the sample cross section in the breaking area:

2 R H = F S m The bonding has to be performed using the special glue, which ensures sufficient strength of the joined part with respect to the coating material and to the production technology. It means that the individual technologies preliminary determine the approximate adhesive strength of the final coating. Thus, the high pressure / high velocity oxygen fuel spray technology (HP/HVOF) guaranties a higher coatings adhesive strength than e.g. the plasma spray technology. For illustration, the adhesive strength of the HVOF coatings is approximately about 90 MPa and for the plasma spray coatings it is about 30 MPa. It is given especially by the coatings microstructure, which strongly depends on the used spray technology. The choice of the adhesive medium (glue) for the adhesive test relates with this fact. For porous coatings it is necessary to choose the glue with minimal penetration properties and simultaneously with adequate tensile strength. Whereas, for dense coatings with minimal porosity the glue penetration is not an important factor it is necessary to ensure the adequate tensile strength of the glue. The bonded part is then cured in the furnace during a defined time Fig. 1. Evaluation of the and temperature. The failure measurement of the samples is then performed adhesive strength on the tensile machine at the load velocity of 2mm/min. After the rupture the occurrence of the glue, the coating or the interface failure is evaluated. If the glue failure appears it means that the coating adhesive strength is higher than the strength of the adhesive, if the coating failure appears it means the cohesive failure and if the failure in the coating and substrate interface appears it means the adhesive failure. It can happen that the failure is the combination of the last two mentioned cases. In such a case it is the adhesive-cohesive coating failure and the ratio of the adhesive and the cohesive failure is determined from the fracture area [3]. 1.1 Special tool for samples coaxial fixturing The axial fixturing of all parts is the main requirement for good adhesive bonding of the samples to the bolstering and loading members. Regarding this requirement the special tool was manufactured. Using this tool the coaxial bonding is guaranteed and using the screws placed on the front sides of the tool the required load of the bonding parts is ensured. The picture of the tool is shown in Fig. 2. The tool was manufactured of the thermally resistant material to prevent from the dilatation or deformation of the tool during curing. The tool is composed of two symmetrical parts, which are Fig. 2. Special tool for joined together with four screws. The slot for sample and members placing is samples coaxial fixturing milled in the centre of both symmetrical parts. In the middle of the slots two channels for adhesive (glue) outflow are placed. The sample covered on one front surface with the coating is put on the bridge that is placed between two outflows channels. Using the screws both members (loading and bolstering) are pressed to the sample. On the front surfaces of the tool the scrolls for the M16 screws are threaded. By tightening of the top screw the movement of loading member to the sample is ensured. The down screw prevent the bolstering member from moving in axial direction, and thus from the movement of the sample. The roughness of the members has to be maximally 0.5 um Ra to ensure members movement in the tools slot.

3 1.2 Adhesives For the ensuring of high strength of the joined parts it is necessary to use the glue of tensile strength that is higher than the bond strength of the coating. For the adhesive tests the HTK Ultra Bond 100 adhesive was chosen. This special glue was produced specially for the tensile test of thermally sprayed coatings. The HTK Ultra Bond 100 glue belongs to liquid adhesives. The liquid is able to penetrate into the coating and thus provide a better bond and the coatings tensile strength rises [4]. It must be taken into account and in case of testing very porous coatings a film adhesive, for example FM 1000 film adhesive with high tensile strength must be used. Further requirement for the excellent sample bonding is to ensure required loading of the joined parts. The calculation of axial torque for screwing up in the tool is following [5]: M z = 1 Ph + πd 2 f z d 2F0 2 πd P f 2 h z It holds for the tool: axial force F 0 =350N, screw M16, lead of screw thread Ph=2mm, average bolt thread diameter d 2 =14,7mm and friction coefficient in thread f z =0,35 (non-lubricated). The calculated value is 1,03N.m after substitution. Resulting value of the M z is the torque value that is set on the wrench. This setting ensures required loading of the bonded parts. The magnitude of loading force is referred to by the producer of adhesives for each adhesive type. 2. EXPERIMENT AND TESTED MATERIALS The plan of experiment was divided into two basic phases. In the first phase the adhesive medium of the HTK Ultra Bond 100 was evaluated for finding the maximal tensile strength of the glued joint. The influence of the roughness of the surfaces of loading members and the influence of the size of the tightening torque were investigated. The second experiment phase included the dependence of the coating thickness on the adhesive-cohesive strength of the coating. Two thermally sprayed coatings were evaluated: HVOF sprayed coating of WC-Co/NiCrFeBSi and flame sprayed coating of Inconel 718. The WC-Co/NiCrFeBSi coating is widely used as the protection against erosion and abrasion and it is used for glass mold plungers, pumps, plungers, sleeves, extrusion screws, steel mill rolls, etc. The Inconel 718 is harden-able nickel-based alloy designed to present exceptionally high yield, tensile and creep-rupture properties at temperatures up to 1300 F. The Inconel 718 has excellent weldability compared to the nickel-base superalloys, which are hardened by aluminium and titanium. This alloy are used e.g. for jet engine and high-speed airframe parts such as wheels, buckets, spacers, and high temperature bolts and fasteners. The chemical composition of the coatings is mentioned in Tables 1 and 2 and the coatings thickness in Table 3. Table 1. Chemical composition of the WC-Co/NiCrBSi (50/50) thermally sprayed coating Element WC Co Cr B Si Fe C Ni WC-Co Wt.% Bal NiCrFeBSI Wt.% Bal. Table 2. Chemical composition of the Inconel 718 thermally sprayed coating Element Ni Cr Fe Al Mo Co Ti Others Wt.% Bal.

4 Table 3. Coatings thickness Set A B C D E F G H E1 F1 WC-Co/ NiCrFeBSi Inconel RESULTS 3.1 Test of tensile strength of bonded joint The results of test of tensile strength of bonded joint are shown in Fig. 3. The roughening of the surfaces was performed by grit blasting and used tightening torque was 1N.m and 2N.m. From the results it can be seen that the surface quality and the magnitude of the tightening torque (loading force) significantly influence the tensile strength of the bonded joint. For each set the tests were performed on 3 samples and the showed results are the average values. Fig. 3 shows that the tensile strength of the bonded joint for non-gitblasted surfaces is much lower than for the surfaces, which are roughened. The influence of the tightening torque is considerably visible only Fig. 3. The strength of bonded joint in dependence on for the non-grit-blasted surfaces. The difference the surface roughness and on the tightening torque between blasted and non-blasted samples is about 25 N.m. For the tensile tests of the selected coatings the follows tests parameters were chosen: gritblasted surfaces (except the front side of the test samples covered with coating) and the tightening torque of 1N.m. 3.2 Coating bond strength in dependence on the coating thickness The coatings thickness is showed in Table 3. For the Inconel 718 coating it was not possible to spray the whole range of the required coating thickness regarding to very low deposition efficiency. Only two types of samples were sprayed, the E1 set (0.2 mm in thickness) and F1 set (0.4 mm in thickness). The samples were subjected to the tensile test and the tests results are shown in Fig. 4. The results of coatings tensile testing do not explicitly show the dependence of the coatings bond strength on the coating thickness as e.g. Godoy [6] mentioned in his work. Most of the WC-Co/NiCrFeBSi tested coatings were cohesively damaged. The adhesive-cohesive behaviour was observed only in minimum cases. The cohesive failure was only local and any detaching of whole self-contained coating did not occur. This behaviour indicates that the coating adhesive strength is superior to the coating cohesive strength. The bond strength between individual structure parts (splats and phases) is lower than the bond strength of the coating to the base material. The interlayer of NiAl, which is generally sprayed under the flame sprayed coatings, supports the bond strength of the coatings. However, all samples of the Inconel 718 coating of the F1 set were adhesively-cohesively damaged. For samples of the E1 set only the glue failure was observed at 61 MPa. That means that the coatings of the set E1 have higher bond strength than the mentioned value. From Fig. 4 it is evident that the bond strength of the Inconel 718 is higher than of the WC-Co/NiCrFeBSi. This fact relates with the coatings properties, which contrast with the used spray technology. Generally, the HVOF sprayed coatings are

5 characterized by higher bond strength to the base material than the others. It is given by the high kinetic energy of the melted particles which impact to the substrate (base material). 3.3 Character of the coating failure The macro image of the WC-Co/NiCrFeBSi coating damage after the tensile test is shown Fig. 4. Dependence of the bond strength on the coating thickness in Fig. 5. The character of the coatings fracture (fracture mechanism) was determined using an electron microscope in Fig. 5. Illustration of the coatings failure after the tensile test: A NTC Centrum Plzeň, CZ. The SEM micrographs were adhesive failure, B cohesive failure, recorded in the BSE (back scattered electron) mode, which C glue failure means that the individual structure elements are scanned in their atomic weights in the grey scale and thus it is possible to recognize individual elements or phases. The SEM micrographs of the coatings failure are in Figs. 6 and 7. Fig. 6 shows the failure in the WC- Co/NiCrFeBSi coating. The substrate material, material of the interlayer (NiAl) and the coating composed of two main phases are clearly evident there. In this case the main failure mechanism was the inter-splat failure. The stress during the coating tensile loading is concentrated to the areas where the weak bond occurs, e.g. the boundary lines between individual splats. In the boundary-lines also the brittle oxides, pores and the un-melted particles occur. These negative coating properties support the cracks propagation and decrease the coating stability. Similar coating failure was observed also for the Inconel 718 coating. The failure mechanism of the Inconel 718 is shown in Fig. 7. In the SEM micrographs the coatings microstructure is also clearly visible. In the fracture the individual splats and their boundary lines are visible, which is the evidence of poor splat bonding during spraying. Nevertheless, the bond strength of the coating is relatively high as the results of the test show. Fig. 6. Inconel 718: the failure after the tensile test Fig. 7. WC-Co/NiCrFeBSi coating: the failure after the tensile test

6 4. CONCLUSION In the presented study the test method for the determination of the tensile adhesive/cohesive strength of thermally sprayed coatings was introduced. The basis of this method is in the tensile test performing and in correct axial bonding of the samples covered with the tested coating. The special tool for axial samples bonding was introduced together with other several important factors. Especially, the types of adhesive medium (glue) and tightening torque were taken into account. Further the tensile strength of the bonded joint was tested. It was found that the roughening (grit-blasting) of the bonded surfaces and setting of optimal loading force (tightening torque) are necessary for ensuring the highest tensile strength of the bonded joint. Two types of thermally spayed coatings differing in the used spray technology were tested and the influence of the coating thickness on the magnitude of the coatings bond strength was evaluated. However, no significant relationship between these two factors was found. Both coatings were fractured above all adhesively-cohesively due to the stress initiation in the splats boundaries during tensile loading. The results of this study are beneficial to the optimization of the methods determining the tensile adhesive strength of the thermally sprayed coatings. ACKNOWLEDGEMENTS This contribution was written thanks to the project No. MSM REFERENCES [1] ČSN EN 582, Thermal spraying Determination of tensile adhesive strength. Czech Institute of Standard, Praha, [2] ASTM C633-01, Standard Method of Test for Adhesion or Cohesive Strength of Thermal Spray Coatings, American Society for Testing and Materials, Philadelphia (Pennsylvania, USA), [3] ENŽL, R. Vysokorychlostní nástřik povlaků na bázi karbidu wolframu, Dissertation thesis, West Bohemian University, Faculty of Applied Science, Plzeň, [4] EVANS, K. A.: Tensile bond Strength Variance of Thermally Sprayed Coating with Respect to Adhesive Type. Proceedings of International Thermal Spray Conference ITSC 1996, C.C. Berndt (Ed.), Published by ASM International, Materials Park, Ohio- USA, 1996, pp [5] BOLEK, A., KOCHMAN, J. Části strojů, 1. volume, 5. printing, Praha: SNTL, 1989, 225 s. [6] GODOY, C., et al.: Correlation between residual stresses and adhesion of plasma sprayed coatings: effect of a post-annealing treatment, Thin Solid Films, Vol , 2002, p