RELATION BETWEEN COMPOSITION, MICROSTRUCTURE AND CUTTING TOOL PERFORMANCE OF ALPHA-BETA-SiALONs

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1 RELATION BETWEEN COMPOSITION, MICROSTRUCTURE AND CUTTING TOOL PERFORMANCE OF ALPHA-BETA-SiALONs N. Ingelström, T. Ekström To cite this version: N. Ingelström, T. Ekström. RELATION BETWEEN COMPOSITION, MICROSTRUCTURE AND CUTTING TOOL PERFORMANCE OF ALPHA-BETA-SiALONs. Journal de Physique Colloques, 1986, 47 (C1), pp.c1-347-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1986 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 Colloque C1, supplbment au n02, Tome 47, f6vrier 1986 page c1-347 RELATION BETWEEN COMPOSITION, MICROSTRUCTURE AND CUTTING TOOL PERFORMANCE OF ALPHA-BETA-SiALONs N. INGELSTROM and T. EKSTR~M AB Sandvik Hard Materials, Ceramic Materials Project, S Stockholm, Sweden RGsum6 - De nouveaux materiaux de type sialon 6laborGs a partir de nitrure de silicium ont fait leur apparition et prgsentent des caractgristiques prometteuses. Des composgs contenant diffsrents taux de la phase w- sialon et de la phase p- sialon peuvent Gtre obtenus en utilisant l'yttrium comme agent de frittage. Cependant il est ngcessaire d'employer des techniques de frittage HIP pour obtenir des structures denses prssentant des taux faibles en aluminium ou en yttrium. On montrera l'influence de la microstructure et de la composition des phases sur les proprigtgs des matgriaux sialons et leur performance de coupe dans la fonte et les alliages r6fractaire.s. Abstract - Based on silicon nitride, sialon materials have been made, which exhibit very promising properties. By using yttria as sintering aid and by changing the overall composition sintered materials with different amounts of d-sialon phase and p-sialon phase can be formed. In order to obtain dense structures also with low A1 and Y content "sinter-hip" techniques have been used. It is shown how the properties of sialon materials as well as the metal cutting performance in cast iron and heat resistant alloys are dependent on microstructure and phase composition I - INTRODUCTION During the past considerable attention has been devoted to the system - SiO - A1 0 - A1N and compositions near the Si Nq Z;Z%~ /1,2?. wit8 b e use of sintering aids, such as yztria a fully dense sialon ceramic body is formed during sintering comprising crystalline components and a glass binder. Depending upon the overall composition a/%-sialon phase or a mixture of d+p - sialon phases will be formed. The binder phase is either amorphous (glassy) or recrystallized. The amount of binder is dependent of the quantity of sintering aids. By using "sinter-hip" techniques /3/ in which the samples are sintered at a high nitrogen pressure, it is possible to obtain fully dense materials with low alumina or yttria additions. This is done in order to reduce the amount of glass phase. The sialon ceramics have many interesting properties which have made them candidates as engineering materials. Today they are widely used as metal cutting tools and as wear parts. By changing the composition and process parameters the properties of the sialons can be altered in order to fit different industrial applications. Article published online by EDP Sciences and available at

3 2 - EXPERIMENTAL The samples have been produced by mixing appropriate amounts of the raw materials Si N4, A1 0, A1N and Y203, see table 1. In or'der to avoid degradatioa of thg AlN, alcohol (water free) was used as milling liquid. In table 1 the compositions are given as equivalent-% which is defined as: Table 1. Composition of grades - Grade eq % A1 eq % 0 c Y A B C D A B C D After milling, drying and granulation, a powder was obtained which was used for compaction of blanks. In a first series the sintering was performed between 1700 and 1800~~ for 2 h in nitrogen atmosphere. In a second series sinter-hip was used at 1725~~ for 1 h in N at a pressure of 200 MPa using a glass encapsulation techniquz /3/. 3 - MICROSTRUCTURE The main crystalline phases are El-sialon, Y (Si,Al) (0,N)16, and p-sialon, Si A1 0 N. The x-value is ty$ically d?2 and z varied between 0 an ge8-gable 2. As can be seen from the XRD measurements of the normally sintered samples, the d/p sialon phase ratio increaseswith increasing addition of AlN, eq-% A1 in table 1. Examples of the microstructure are shown in Fig 1 to 3 using the SEM in backscattered mode (Fig 1) or secondary mode (Fig 2 and 3). There are two classes of sialon grains in the structure: The smaller grains, about one micron large and below, often equiaxed and the larger rodlike grains up to 10 microns. Both types of grains can be of d-sialon (light grey) orp-sialon (dark grey in Fig 1). The glass phase and the b-phase are both light. The amount of glass phase decreases slightly with increasing oc/@ ratio. The grain size varies somewhat within the investigated temperature range. However, the most significant effects are that the porosity level increasesand the density decreasesat sintering temperatures near 1700~~~ see also /4/. The use of sinter-hip does not change the microstructure. However, this technique makes it possible to obtain materials with no substitution of alumina into the p-si N lattice ( z = 0) and still maintain the favourable rodlike struc$u$e as material lc, see Fig 2. In grade 2D, however, where the yttria and alumina content is low the glass content is very low. There is not enough glass present at the sintering temperature and therefore small, virtually equiaxed grains are formed, Fig 3. In the other grades the glass content is ~01%. 4 - PROPERTIES Hardness and indentation fracture toughness - K' - are listed in table 2. In grade 1B-1C-1D the fracture toughness decreases with increasing d-sialon phase content. This decrease is associated with an increase in hardness. From the second series it is seen that sinter-hip may increase (grade 2C) or decrease (grade 2B and 2D) the

4 Fig 1 Material IC Fig 2 Material 1C Fig 3 Material 2D Above SEM micrographs of a polished and carbon coated sample (Fig 1) and samples etched in molten alkali carbonate (Fig 2 and 3) are shown. toughness. Important features to obtain high toughness and hardness are amount of glass phase, compare grade 2C and 2D (with very low amound of glass), glass phase composition and z-value (grade 2B and 2C) and aspect ratio of the sialon phases. Table 2. Properties of the samp1es.crystalline phases in wt-% by XRD Grade d-sialon (3 -sialon b-phase others HVlO - K' 1A - 98,z= B 15 84,z= C 30 69,z= D 50 45,z= H; A 98,z= B 100,z=O C 100, z=o D - 95,z=O - Si2N20; The hot hardness of the materials in r the first series is shown in Fig 4. 5 I By comparison with an alumina+4% zirconia grade it is evident that the 1500 sialon materials maintain a high hardness at high temperatures. This is especially the case for grades with a high d-sialon content. 5 - CUTTING PERFORMANCE The edse and Bulk fracture tests were done as intermittent cutting without coolant. The criterion for tool life was number of passes per edqe. The turninq tests Temperature, C Fig 4 Hot hardness of sialon materials were perfo;med as longitudinal-turning. The cast iron was machined without coolant, but the heat resistant alloys were used with flood coolant. Insert style used was SNGN with a chamfer of 0.2 x 20 for the cast irons and an edge rounding of about 50 microns for the heat resistant alloys. Examples of wear types are shown in Fig 5.

5 Fig 5a, 5b Examples of wear when machining Inconel 718. Cast iron Some preliminarv results are summarized in Fis 6. As can be seen there is not a simple-relationship between properties such as hardness and fracture toughness and cutting performance. For example, the flank wear decreaseswith hardness, grade 1A and ID, but a more important feature seems to be the glass phase composition and the z-value, grade 2A and 2C. Other important features, such as the amount and type of slag inclusions in the workpiece material and the presence of casting skin are under investigation. 0 Edge fracture test v= 800 m/ki n ES Bulk fracture test s= 0.3 mm/rev a= 20 mm 2.- % L mm A, R1 aj ;:; 0, Y -0 c P) m d P) 5 a- 50 C, m 100 P) 0.5 ' E * ZA ZB 2D 1B I D min Fig 6 Fracture and wear behaviour when machining cast iron with sialon materials. The edge fracture test was done with speed, v=500 m/m, feed, s=0.25 mm/rev, depth of cut, a=2 mm and the bulk fracture test with v=300 m/min, s=0.5 mm/rev'and a=2 nun. Heat resistant alloys Two different nickel based alloys have been used for the tests: Incoloy 901 (35 Fe, 13 Cr, 6 Mo, 2.5 Ti) solution treated to HB=300 and Inconel 718 (19 Fe, 19 Cr 3 Mo, 5 Nb) solution treated and aged to HB=450. The results are shown in Fig 7. The criterion for tool life was either severe rake face flaking (more than 50% of the depth of cut), high mean flank wear depth (more than 0.5 mm) or high depth of cut notch wear (more than 2.0 mm), Fig 5. The tests were performed at two cutting speeds, see Fig 7. The tool life for the datum material pd ZC

6 (=loo%) was between one and three minutes for Incoloy 901 and two to five minutes for Inconel 718. The incidence of rake face flaking has not been assessed for grade 2C due to its high overall wear characteristics, Fig 7. INCOLOY 901 The life length when machining Incoloy 901 increaseswith hot hardness and o(-sialon phase content. As can be seen it is essential that at least some d-sialon is present. Both grades 1A and 2C with no o(-sialon show low tool life. The limiting factor has been the incidence of rake face flaking in most cases. INCONEL 718 Inconel 718 is a tougher material to machine. There is no obvious relation between tool life and d-sialon phase content. In order to get a long tool life both toughness and high hot hardness are needed. The resistance to rake face flaking is also here an important factor, especially at high machining speeds. d % 0 n L d 200 m 0, >I.F +C m v A+ 2: A IBIC ID 2C 1A 18 1C 1D2C 1A 1B 1C ID 2C 1A 18 1C 1D2C Fig 7 Wear behaviour when machining heat resistant alloys. Inconel 718 was used at v= m/min and Incoloy 901 at m/min. The feed rate was 0.15 mm/rev and the depth of cut 2.0 mm. 6 - CONCLUDING REMARKS The sialon cutting tool materials have given very good results compared to other ceramic materials when machining nickel based heat resistant alloys and cast irons, especially when intermittent operations are involved. However, there is no simple relation between microstructure, physical properties and cutting tool performance. In the tested workpiece materials, several different wear mechanisms operate. Thus for optimum cutting conditions several different features must be considered. Compared to alumina based ceramics the sialons exhibit very good toughness behaviour but the wear resistance is lower. However, in certain machining operations a protective layer (TiN, A1 0 ) has been observed on the cutting tip which decreases the wear rat2 possibly a transition from dissolution to diffusion

7 controlled wear. It can be concluded that a high aspect ratio grain structure is favourable when machining cast iron. In addition, a high Wp-sialon phase ratio and thus a high hot hardness is beneficial when machining heat resistant alloys. This is discussed in more detail in (5). REFERENCES 1 K H Jack, Metals Tech 9 (1982) C Chatfield, T Ekstram, M Mikus, J Mater Sci, in print 3 H T Larker, Emergent Process Methods for High-Technology Ceramics, Vol 17 (1984) T EkstrBm, N IngelstrBm, "Non oxide Technical and Engineering Ceramics", Conf Ireland July, J Aucote, SR, Foster, Materials Science and Technology, in print.