THE KNOWLEDGE BASES FOR SELECTING THE SURFACE MODIFICATION TREATMENT T.Filetin, D.Novak, M.Stupnisek Faculty of Mechanical Enginering and Naval Architecture, University of Zagreb, I. Lucica 5, HR-10000 Zagreb, Croatia; e-mail: tfiletin@fsb.hr Abstract: The main influence factors on the type and intensity of tribological wear are: construction of tribological system, the properties of substrate material and surface layer, and the working parameters. Because of that, wear problems are usually very complex for solving. Another obstacle is the lack of quantitative and comparable data on wear resistance of materials. Data, information and knowledge needed for decision making are mostly qualitative and come from experts and from the exploitation of the tribological systems. Knowledge bases are the basic modules in the development of the expert system for solving the tribological problems. The following knowledge bases, related to particular surface modification treatments, are defined and established: the technical characteristics of the treatment - description, achievable properties; the characteristics about the base materials (substrata); commercial characteristics of the treatment the investment and labour cost, manufacturers and users of the equipment; the properties of surface treated and exploited tribological workpieces; description and photos of worn out workpieces; the standards for determination of the layer depth and the methods for testing wear resistance; the microstructures of the surface layers. Besides the traditional heat treatment processes - carburising, nitriding, boriding, surface hardening etc, many new surface modification processes in today's use are recommended in order to decrease wear intensity. For now, the described surface modification treatments come from the following groups: treatments with the martensitic transformation, treatments with the modification of chemical composition, thermal spraying, PVD and CVD, weld hard facing, laser modification. Browsing, searching and linking between particular modules of this expert system are enabled. Key words: surface modification treatment, selection of treatment, expert systems 1. Introduction In solving wear problems two ways are possible: the choice of wear resistance through the whole cross section, or application of the wear resistant surface layers. The development in surface engineering processes resulted in growing up of new technologies for increase of wear resistance, corrosion resistance and for aesthetic function. Besides the traditional processes surface hardening, carburising, nitriding etc, a number of advanced physical technologies being increasingly used, like the laser and plasma assisted processes. MT2002 1
For engineers especially in design and in definition of production processes, the right choice of appropriate surface treatment is a very important task. Nowadays in this field, a lack of adequately prepared quidance and expert proposals is evident. Data, information and knowledge needed for decision-making are mostly qualitative and come from experts and from exploitation of the tribological systems. Computerised system offers new possibilities for collecting, storing, searching and comparing great amounts of information (data and knowledge) derived from different sources literature, own investigation and experience from the use of tribosystems. Data and knowledge bases like the basic modules of the expert system and technique of hypertext, hypergraphic, frames and expert rules enable linking between particular modules and help solving tribological problems. In this paper, the structure of the expert system and its knowledge bases on the surface modification treatment are presented. 2. Description of the expert system Figure 1 shows the basic structure of the expert system for selecting the surface modification treatment. Figure 1. The data bases of the expert system for selecting of surface modification treatment. MT2002 2
The main modules - one for the analysis of tribosystem and the other for choosing the appropriate treatment in conversation with users, enable the analysis and diagnostics of wear problem. In the second step they propose solutions for the use of suitable surface treatments. An inference engine manipulates with stored data and knowledge by means of expert rules. Data base queries, frame technique, hypertext and hypergrahics permit classification and searching of wear cases, previously treated parts, and surface modification treatments which are most similar to the tribosystem to be solved. Nowadays, the development of this expert system is focused on organisation and storing of knowledge. The defined data bases, knowledge bases and their relations are illustrated at fig. 1. - Technical characteristics of different surface modification treatments. The knowledge base nowadays contains description of 40 technologies (Kunst [1]); - Damaged tribo-elements from exploitation knowledge base contains over 100 case studies from literature and our own investigation for the industry (Lisjak [2] and Evert [3]); - Wear testing results: comparable data on wear resistance of different surface layers (Neale [4] and COST 516 [5]); - Related standards for determination of depth and hardness of surface layers, as well as descriptions of testing methods of wear resistance properties. The suggested structure of the particular knowledge bases is: DB1 Technical characteristics of the treatment contain the following items: - Short description of treatments; - Constructional conditions; - Treated materials, surface conditions, expected hardness and other properties, recommended depth of layer; - Description of treatments variants. The knowledge base contains descriptions of surface treatment technologies from the following groups: 1. Treatments with martensitic transformation hardening of whole cross section, flame hardening, induction hardening, laser hardening, electron beam hardening, carburising; 2. Treatments with modification of the chemical composition nitriding, carbonitriding, boriding, vanadising; 3. Thermal spraying self-flowing alloys, pure metals, metal carbides, metal oxides, alloys, other materials; 4. CVD high temperature CVD layers with Ti base, high temperature CVD layers with Al 2 O 3 -base, medium temperature CVD layers with Ti(C,N) base; 5. PVD TiN, TiN&TiC, TiAlN, CrN layers; 6. Weld hard facing with steels similar materials, hard carbides, corrosion resistant layers, corrosion and wear resistance layers, dispersion alloying layers; 7. Laser modification (melting) surface alloyed layers, laser cladding. MT2002 3
The main source of information on the different treatments was (Kunst [1]). DB2 Commercial characteristics of the equipment and process - Manufacturers of equipment; - Investment and labour costs; - Reference installation; - Other information. DB3 Description of the previously treated parts - Description and picture (photo) of treated part; - Working condition in use; - Substrate material and their condition; - Applied surface modification and other treatments; - Depth of layer and hardness; - Other properties after treatment. DB4 Collection of the damaged tribo-elements from exploitation - Base material; - Description of tribosystem and damaged part; - Type and wear mechanism; - Photo(s) of damaged place; - Working condition; - Wear causes; - Means of solution of wear problems. The collected and stored parts of tribo-elements belong to the following groups: gears, sliding and rolling bearings, shafts, pipelines, components of pump, components of steam and gas turbines, motors, impellers, cutting tools etc. DB5 Microstructures of the treated parts The files contains photos of typical microstructure of the surface layers and substrates with possible microstructure defects. DB6 Results of testing the tribological properties of the surface layers The data base contains the following items: Testing method or standard; specimens base material, applied treatment, depth of layer, hardness; testing parameters; results of testing. The wear properties data of surface layers are derived from COST 516 programme Tribology (COST 516 [5] and 2 nd COST 516 [6]) and other own laboratory investigations of abrasion, adhesion and impact erosion resistance. MT2002 4
3. Selection of the appropriate wear protection treatment Figure 2 shows the simplified decision-making flow process for choosing an appropriate surface protection treatment. Figure 2. Scheme for decision making on the appropriate surface treatment At the beginning it is necessary to analyse the functional, technological requirements and expected working condition of tribosystem. The probable type and mechanism of wear, required hardness and layer depth result from this consideration. In finding the most possible wear behaviour helps the base relations, given in the table 1, between the type of triboelements, relative motions, mode of wear, and the preferable wear mechanism and protection technologies on the other hand. Additionally, the reviewing and browsing through existing knowledge bases on worn out parts from the use, as well as databases on treated parts indicate a satisfactory wear resistance in exploitation. The expected type and wear mechanism and the stored data with expert rules support preselecting the variants of appropriate surface modification treatment. Example: The elements of tribosystem are: a worm extruder that presses sunflower seeds which contain hard particles. The relative motion is sliding. The expected type of wear is abrasion and the expected wear mechanisms are: abrasion and tribocorrosion (possible). Solutions (results): The recommended useful surface modification treatments are (table 1): - thermal spraying of: self-flowing alloys, pure metals, alloys, metal carbides or metal oxides, - laser remelting: embedded dispersed particles, laser cladding, - boriding, - vanadising. MT2002 5
Table 1. Assignment of the type of triboelements, relative motions, mode of wear, and the preferable wear mechanism to specific surface modification treatment MT2002 6
After preselecting of the preferable treatments, the technical and commercial characteristics of these technological processes could be compared. For a final decision and determination of the technological parameters as well as the elements of quality layers depth, microstructure, surface roughness, hardness and other properties, browsing and reviewing of the other existing knowledge and data bases are needed. Figures 3 to 5 illustrate some steps in the surface treatment selection for the previous example. First computer screen (figure 3) shows input characteristics - the type of triboelements, relative motions, the expected mechanism of wear, and preselected surface treatments. In the second step it is recommended to see stored characteristics of preferable processes (figure 4). The screen at fig. 5 shows a picture of worm press from exploitation with working parameters. Figure 3. Input characteristics for abrasion type of wear and preselected surface treatments MT2002 7
Figure 4. Stored characteristics for vanadising process Figure 5. Picture of worm press from exploitation with working parameters and wear characteristics MT2002 8
4. Conclusion Lack of reliable and comparable quantitative data about wear resistance of different surface layers, as results of surface treatments, is the main problem in determination of suitable surface protection. The collection, validation of experimental data and expert knowledge, formation of knowledge bases on wear resistant materials and surface modification treatments are the most demanded issues in development of the expert system for solving tribological problems, especially for choosing the appropriate surface protection mode. The corresponding data bases and knowledge bases about surface modification, treated parts, case studies and wear testing methods are constituted, which is a good frame for their future supplementing, enlarging and improving. The future activities are related to the definition of quantitative comparable searching criteria, logical expert rules and procedures which enable linking between particular modules of the expert system and help decide on the appropriate surface treatment for protection of tribosystem in the real situations. References: 1. Kunst H. Verschleiβschutz - Ein Ratgeber für die Anwendung verschleiβhemmender Schichten, Arbeitsgemeinschaft Wärmebehandlung and Werkstofftechnik (AWT), Wiesbaden, 2000. 2. Lisjak D. Development of the expert system for solving the abrasion wear problems, (In Croatian), Master Thesis, FEMA, Zagreb, 1998. 3. Evert DD, During A. Corrosion Atlas Collection of illustrated Case History, Elsevier Publ., 1997. 4. Neale MJ (Ed).Tribology Handbook, Butterworths, London,1973. 5. COST 516 Tribology Symposium Proceedings, Technical Research Centre of Finland, VTT Espoo, Finland, 1998. 6. 2 nd COST 516 Tribology Symposium Proceedings, Flemish Institute for Technological Research, Antwerpen, Belgium, 1999. 7. Filetin T, Lisjak D. Development of the expert system for selecting of the surface modification treatment, Euromat 99, München, 1999, Proceedings Vol. 11 - Surface Engineering, Wiley-VCH & DGM, p. 343-349. 8. Filetin T, Lisjak D, Novosel M, Markovic R. Podloge za izbor postupka modificiranja povrsina (in Croatian), Proceedings of the MATRIB 99 Conference, Trogir, 1999. s.61-68. 9. Stupnisek M, Matijevic B. Pregled postupaka modificiranja i prevlacenja povrsina (in Croatian), Proceedings of the Conference Heat Treatment and Surface Engineering, Zagreb, 2000. s. 53-62. MT2002 9