AUTOMOTIVE APPLICATIONS. Testing Instructions

Transcription

1 AUTOMOTIVE APPLICATIONS Issue date: A Testing Instructions Designation Reference Coverage Scope GOE 504 B 14 GOE 505 B 15 GOE 520 B 24 quality testing (all material specifications) quality testing (all material specifications) thermal spray coatings (GOE , 210) modulus of elasticity bending strength diametral force hardness testing bending strength structure testing thermal testing hot hardness preparation coating characterization coating hardness GOE 530 B 27 electrochemically deposited hard chromium (GOE ) preparation coating characterization coating hardness

2 Reference: B 14 Material Specification Piston Rings GOE 504 Issue date: A Quality Control of Piston Rings - Formulae - F modulus of elasticity E in MPa E = t ( d -1) 3 a *) m. h F t = tangential force in N d = ring nominal diameter in mm a = radial wall thickness in mm h = axial width in mm m = free gap in mm *) valid for rectangular rings F bb F bb (6d - 5a) bending strength σ bb in MPa σ bb = F bb h. a 2 *) F bb = breaking load in N d = ring nominal diameter in mm a = radial wall thickness in mm h = axial width in mm *) valid for rectangular rings diametral force F d in N related to nominal diameter: F d = 2.05 F t for untreated and hardened + tempered grey cast irons F d = 2.15 F t for nodular cast irons These are empirically determined values. The theoretical correlation between tangential force Ft (measured with flexible steel band) and diametral force Fd at nominal diameter is determined by: F d = 2.21 F t

3 Material Specification Piston Rings GOE 505 Issue date: A Quality Testing of Piston Rings - Test Methods - Hardness Test (as detailed in the applicable specifications) Base Materials: HRB to DIN EN ISO on cast irons, HB to DIN EN ISO on cast irons, HRC to DIN EN ISO on harder materials (e.g. steel), HV 30 to DIN EN ISO 6507/1 on rings with high hardness and small wall thickness, HV 5 to DIN EN ISO 6507/1 on rings with high hardness and small wall thickness. The hardness data are averages of three measurements on a single ring: 1 st measurement at gap; 2 nd measurement 90 from gap; 3 rd measurement diametrically opposite the gap. Material evaluations must be based entirely on measured values; values obtained by conversion are only approximate. Coatings: HV 1 to DIN EN ISO 6507/2: Coating hardness of plasma spray coatings, HV 0.05 to DIN EN ISO 6507/3: Particle hardness of multi-phase spray coatings, HV 0.1 to DIN EN ISO 6507/3: Coating hardness of chromium and molybdenum coatings. Bending Strength Test The piston rings are stretched open tangentially at a rate of between 0.5 and 1.0 m/min until they break. On ductile materials the bending strength cannot be determined accurately using this method. Structure Test Base materials: Cross-section about 90 from the gap, evaluation zone is mainly the outer third of the section, structure is rated according to the applicable specification or the classification chart. Scales of magnification for structural examination: Graphite structure 100 : 1 Matrix 500 : 1 Phosphide eutectic 20 : 1 Thermal spray coatings: see specification GOE 520 Electrochemically deposited chromium coatings: see specification GOE 530 Thermal Test on piston rings: on spring loaded rings: on steel oil control rings: annealing temperature 300 C, time 3 h annealing temperature 250 C, time 5 h annealing temperature 220 C, time 5 h Tangential force loss after heating under installation stress, as per DIN ISO 6621, Part 5. Hot Hardness Annealing temperatures: 100, 200, 300, 400, 500, 600 C Hardness indentations HB 2.5/187.5 at each of the annealing temperatures, measurement of indentations at room temperature, hot hardness stated as the average of 6 tests at each temperature level.

4 Reference: B 24 Material Specification GOE 520 Issue date: A Testing Guidelines for Spray Coatings on Piston Rings For determining the material data of spray coatings on piston rings the procedures employed in specimen preparation and testing are vitally important. It is therefore necessary to establish testing guidelines for spray coatings, so that reproducible material data can be obtained. Quality evaluations of spray coatings are permissible only on non enginerun rings; examinations of engine-run coatings primarily yield information on performance in engine operation. Spray coatings are tested by destructive and non-destructive methods. Destructive testing comprises an analytical test (e.g. chemical analysis) and a metallographic investigation. The non-destructive procedure consists, for example, of visual inspection, surface crack testing and measurement of the coating thickness. 1 Metallographic Preparation The specimens used in the metallographic examination of spray coatings and in hardness testing require meticulous preparation. The procedure to be used depends on the type of spray coating. For plasma, flame and HVOF coatings the following procedure is standard: 1.1 The ring of interest is cut radially at several points around the circumference using an abrasive wheel. In order to prevent the ring from heating at the cutting point, a wet cutting process must be used and the rate of advance should not be too fast. The cut must always be made working from the coating towards the base material. 1.2 The cut ring segments are mounted on-end in a plastic compound such that the coated running face of each segment rests as tightly as possible against the inside face of its neighbouring segment (see sketch below). The ring pieces may be lined up in several adjacent rows. To protect the spray coating of ring segments which are so large that they can only be mounted singly, a piece of cast iron or steel is placed in front of the coating and is cast in together with the ring segment. For reasons of economy, hot mounting is often preferred to cold mounting. In the case of plasma and flame sprayed coatings the specimen temperature when using a hot mounting press should not exceed 140 C. HVOF sprayed coatings may be mounted within a range of 160 C-170 C. Generally it is useful to allow a warm-up phase of 2-3 mins before pressing. After removing the specimens, they must be clearly inscribed on the back or outer surface!

5 1.3 The specimens must always be ground such that the abrasive grain presses the spray coating against the substrate metal. Whenever possible an automatic preparation machine should be used in preference to manual preparation. It should be remembered that the polishing agents and cloths supplied by different manufacturers vary in quality and the quality of the cloths changes with length of use. Where there is doubt, polishing agents and new cloths from the same manufacturer must be used and the polishing must be carried out on an automatic machine. The procedure for plasma and flame sprayed coatings is as follows: First, 2 to 3 mm of the specimen should be removed by wet grinding in order to take off any damage caused in the cutting operation. This rough grinding is performed with 60 or 80 grit (average grain size 260 or 196 μm) ensuring that no heat is generated. Rough grinding is followed by the actual grinding procedure using wet-quality silicon carbide abrasive paper in a diameter of 230 or 250 mm in the following sequence: Grit 180 Grit 220 Grit 320 Grit 1000 Grit 1200 The maximum permitted speed of the grinding wheels is 560 rpm up to 1000 grit and 280 rpm with 1200 grit. The specimens must be ground with each grade of paper until the abrasive marks from the previous grinding operation have been removed. 1.4 After superfine grinding, the section must be cleaned in the ultrasonic bath for a short time, max 2 minutes. 1.5 The section is polished with diamond, the following guide data being important for obtaining a mark-free polished section: flame sprayed molybdenum coatings grain size 3 μm, polishing time about 5 mins. metallic and metal-ceramic plasma coatings grain size 3 μm, polishing time about 5 mins, then ultrasonic cleaning and grain size 1/4 μm, polishing time about 7 mins. 1.6 After final polishing, the specimen should first be washed under running water to remove remains of polishing medium and then rinsed in alcohol and dried in warm air. 1.7 The etching of the specimen in order to differentiate different phases is carried out with a suitable etchant. All-molybdenum and high-molybdenum coatings are etched by the Murakami method using an aqueous solution with the following composition: 0.2 g potassium ferricyanide 0.2 g potassium hydroxide 40 ml distilled water. The etchant must be freshly prepared before use and must not be warmed. The procedure for metal-ceramic HVOF spray coatings, while heeding the preceding remarks, is as follows: 1.8 If the section was cut slowly and with cooling fluid it will be sufficient to grind off mm with the wet abrasive wheel (60 or 80 grit) in order to remove any damage caused by cutting. As HVOF coatings are harder and more brittle, this grinding operation must be carried out with extreme care since damage to the surface of the specimen, with effects deeper down, can occur at this early stage.

6 1.9 The subsequent grinding operation covers the following steps in the stated sequence: Grit 220 Grit 320 Grit 800 Grit 1200 Grit 4000 The abrasive wheel speed should be 300 rpm and the contact pressure should be 200 N for 4 specimens. The specimens must be ground with each grade of paper for 45 sec. The 220 grade is used twice, all other grades once. The best grinding results are obtained if the specimen mount contains 4 specimens and the grinding is performed automatically. If necessary dummy specimens should be used to make up the number After superfine grinding, the section must be cleaned in the ultrasonic bath for a short time, max 2 minutes, using e.g. ethyl alcohol Polishing is best performed using napped cloths and observing the following guide data: grain size 1 μm, polishing time about 1.5 min, 150 rpm, 100 N for 4 specimens Etching may be carried out using the same procedure as in 1.7. The concentration of the Murakami formula may vary, in which case the etching times will change accordingly. The chromium carbide-containing phase is not etched, while the tungsten carbide-containing phase is strongly etched and takes on a dark tone in the optical microscope. 2 Quality Characteristics The following quality characteristics of spray coatings are tested: 2.1 Chemical Composition 2.2 Coating Porosity 2.3 Pore Size 2.4 Microstructure and Phase Distribution 2.5 Unmelted Particles and Reaction Products 2.6 Microcracks and Fissures 2.7 Coating Hardness 2.8 Particle and Phase Hardness 2.9 Running Face Porosity, Voids, Cracks, Bond Defects between Coating and Inlay Groove Land (on Inlaid Rings) 2.10 Coating Thickness

7 The following must be considered when testing the stated quality characteristics 2.1 Chemical Composition The spray coating is removed from the ring for analysis by stretching the ends of the ring apart or striking it until the coating comes free. The coating is then crushed. If the coating is contaminated with base material the analysis must be suitably corrected. The analysis is performed using analytical procedures (e.g. AAS) appropriate to the elements being tested. 2.2 Coating Porosity The porosity of spray coatings is evaluated on the unetched microsection. It is important for the evaluation to be carried out on representative areas of the coating. In the case of inlaid spray coatings the areas near to the inlay groove walls are not to be considered as representative because in these regions turbulence is generated in the spray jet during spraying and this can result in greater porosity. The specifications define maximum values for the porosity of representative areas of coating on full-face sprayed, half-inlaid and fully inlaid piston rings. The porosity in the inlay groove wall region may be twice the value of the representative area. The size of the inlay groove wall region y is defined by the function y = cx, where c is an empirically determined constant and x the actual coating thickness. For the inlay groove wall designs used up to now the value for c is The coating porosity can be determined by 2 methods: The pores in the coating areas are measured with a quantitative image analyzer. The surface area of all measured pores is set in relation to the area of the measurement field. The average of 30 measurement fields with a cumulative area of about 1.5 mm² is calculated for each ring The pores in the coating areas are estimated by comparing them against a classification chart containing photomicrographs of known porosities.

8 2.3 Pore Size Pore sizes are stated as a size distribution percentage. All pores in representative measurement fields are measured for size in the unetched section and placed into size classes. The measurements are best performed by quantitative image analysis as in The number of all measured pores is set equal to 100%. The material specifications for the respective spray coatings state the percentage of pores smaller than a specific value. An additional value is stated for the maximum size of individual pores. The specification further states the maximum pore size in the radial direction relative to the coating thickness. 2.4 Microstructure and Phase Distribution The microstructure and the phase distribution of spray coatings are assessed on the etched microsection. As these variables are difficult to quantify, the evaluation is performed by means of comparison against a classification chart for the spray coating concerned. The photomicrographs contained in the specifications represent only averages. 2.5 Unmelted Particles and Reaction Products As a result of the spray process, plasma and flame sprayed coatings contain unmelted or only partially melted spray particles as well as reaction products usually of an oxidic nature. Unmelted particles are recognizable as more or less round inclusions in the coating structure; reaction products can be present as thin layers between the coating lamellae. The permissible size and number of unmelted particles per sectional area is stated. The permissible shape and amount of reaction products is defined with the aid of reference micrographs (classification chart). In the case of HVOF sprayed coatings it is the specific aim not to melt the spray powder but rather to compact the softened particles. Therefore unmelted particles in HVOF spray coatings are not a negative quality characteristic. 2.6 Microcracks and Fissures Microcracks in the structure of spray coatings are short cracks discernible at 100x or greater magnification running between the coating lamellae or transversely across them. Fissures are lengthy cracks within fairly large coating areas or between the coating and the substrate metal. The evaluation of spray coatings for microcracks and fissures is carried out on the unetched microsection. Microcracks are allowable, fissures are not. Note: A ghost line at the coating to base material interface may be caused by relief formation during specimen preparation and will prevent a clear evaluation of the adhesion of the coating to the base material. If there is any doubt, the section must be suitably illuminated at an angle or an SEM micrograph taken in order to discriminate between a ghost line and a genuine fissure. 2.7 Coating Hardness The coating hardness is measured according to Vickers as defined in DIN ISO 4516 and is stated as the average of 10 useful individual measurements per ring. The average must lie within the tolerance stated in the appropriate coating specification. Measurements are carried out on the cross-section prepared as in 1.1 to 1.6 above. If the test area is too small, several sections may be necessary.

9 The test force applied is dictated by the type and thickness of the coating. As the coating hardness is an integral quantity, the test force chosen should be such that the indentation will preferably extend over several spray lamellae or structure phases. The test forces to be applied are stated in the respective coating specifications. 2.8 Particle and Phase Hardness The hardness of individual particles and phases is measured on the etched section usually with HV For very small particles and narrow phases it may be necessary to use a lower test force. In accordance with DIN ISO 4516 the test force is applied with an impact velocity of the indentor onto the specimen of μm/sec. The equipment setting must not be altered for the duration of the test. The test force is allowed to act for 10 to 15 sec during which time no jolts or vibrations must be permitted to interfere with the applied force. The average of 10 useful indentations is taken for each phase. The averages must lie within the tolerances stated in the specification. 2.9 Running Face Porosity, Voids, Cracks, Bond Defects between Coating and Inlay Groove Land (on Inlaid Rings) These features, distinguishable on the running face, are influenced by the coating quality and above all by the machining. Such running face defects are tested by visual inspection, if appropriate with magnification. Guide values are laid down in DIN ISO for the evaluation of porosity and voids in the running face and for assessment of the running face edges and the outer edges at the ring gap. Macroscopic cracks in the running face are not permissible. If there is any doubt, a decision is made based on a suitable crack testing procedure. There must be no bond defects visible on the running face in the form of fissures between the coating and inlay groove land. However, allowance must be made for the occurrence of a partly discontinuous bond as a result of the unavoidably greater porosity of the coating structure caused by turbulence in the spray jet in the inlay groove wall region Coating Thickness The thickness of spray coatings is determined with a device for measuring non-ferromagnetic coatings on ferromagnetic base materials (e.g. Permaskop). Standard reference values for different ring designs (fullface sprayed or inlaid) are obtained based on microscopic coating thickness measurements on radial crosssections. The coating thickness is measured in the middle of the coating at three points around the ring circumference in accordance with DIN ISO and -4. The measured values must correspond to the drawing specification, with permissible tolerances stated in DIN ISO The piston rings must be sufficiently demagnetized prior to measuring. If there is any doubt, the coating thickness must be determined on the radial cross-section.

10 Classification Chart for Porosity Evaluation of Thermal Spray Coatings (Appendix to section 2.2 of the Guidelines) Percentage Porosity approx. 2 % approx. 5 % approx. 10 % approx. 15 % approx. 20 % unetched 200 : 1

11 Reference: B 27 Material Specification Inspection Instructions GOE 530 Issue date: A Guidelines for Assessment of Electrochemically Deposited Hard Chromium Coatings Reinforced with Hard Particles 1 Scope The specification describes inspection procedures for determining parameters of the material specifications GOE 230 (CKS 36), GOE 234 (CKS 37), GOE 231 (CKS 38), GOE 232 (GDC 50) and GOE 233 (GDC 52). 2 Parameters The following parameters are assessed: Microcrack density of the coating containing hard particles Proportion of hard particles in the coating Grain size of the hard particles Coating hardness Structure of the graduated coating For assessment of the stated parameters the following procedures must be observed: 2.1 Microcrack density of the coating containing hard particles The crack density is determined on running face specimens of ring segments prepared according to the following guidelines: Sampling A segment is cut from any desired part of each ring using a wet cutting machine Mounting The cut segments are placed into the mounting mould with their running face on the base of the mould. To protect the coating, a piece of cast iron or steel is placed in front of the segments and cast-in together with them (see sketch).

12 In the case of hot mounting, the specimen temperature must not exceed 170 C. Depending on the size of the ring and specimens, from 1 to 4 segments can be mounted and prepared simultaneously Grinding The grinding must be carried out with SiC wet-grinding paper using the following grades in succession: Grit 220 Grit 320 Grit 600 Grit 1200 Grit 4000 In order to expose the whole structure of the coating the coarsest grade of paper is used until the base material becomes just visible (see sketch) Polishing Polishing is carried out using a 1μm diamond suspension until the specimens are free from scratch marks and the contours of the hard particles are sharply defined Etching For determining the crack density the microcrack network is made more visible by means of electrolytic etching using 5% NaOH at room temperature with a current density of 30 A/dm² Determining the microcrack density Two 50 mm-long diagonally intersecting straight lines are placed in the image window at 100x magnification. The microcrack density is determined from the sum of the number of cracks intersected by the two straight lines. The crack density can also be determined using an image analyzer with a suitable evaluation program.

13 2.2 Proportion of hard particles in the coating The proportions of hard particles in the coating are analyzed by wet chemical means and converted to % by volume. For the conversion from percent by volume to percent by weight the following specific weights must be applied: Cr : 7.0 g/cm 3 Al 2 O 3 : 3.94 g/cm 3 Diamond : 3.5 g/cm Grain size of the hard particles The size of the hard particles is assessed microscopically at 1000x magnification on the unetched specimen. 2.4 Coating hardness The coating hardness HV 0.1 is determined as the average of 5 measurements on the polished running face in accordance with DIN EN ISO 6507/ Structure of the graduated coating The structure of the coating is examined on the cross-section prepared in the same manner as the running face section described in 2.1. The coating thicknesses are measured under the optical microscope at 100x magnification. The measured thickness of the coating containing hard particles is stated in relation to the overall coating thickness. 3 Other coating features 3.1 Running face porosity Microporosity on the running face is typical for the process and the type of coating. Pores with a diameter > 150 μm are not permissible.