Determination of some material properties of borides coated on the surfaces of low alloy and carbon steels

Although it is well known, that Turkey is rich in certain mineral resources, how to use available deposits economicaly is not fixed yet for most of resources. Since one of the important resources, which Turkey has, is boron, usage of which is always considered preferentially. One of the applications of boron or boron based compounds is to get benefit of some of the extereme mechanical properties of boron compounds as coating materials on metallic and non-metallic surfaces with or without solutions hardening through boron in base materials. Coatings are used in many engineering applications to improve surface properties by the help of beneficial properties of components as coating materials. Main general reason of the application of coating techniques is to protect base material from outside effect. Coatings being surface modification techniques are designed to enhance surface mechanical properties, the corrosion resistance in aggresive environment and/or improve friction and wear properties for tribological applications. It is well known, that coating layer differs from substrate in chemical composition, structural properties, and chemical and mechanical behaviour. Coating technologies range from painting, through coatings which are applied than fixed to a surface, to plasma spraying, to various chemical methods of deposition, to an increasingly sophisticated array of vacuum techniques. Painting is the most familiar method of surface coverage, whereas fixed coatings are relate to traditional enamelling in that the surface coating contains not only inorganic oxides such as alumina, which provide wear and heat resistance, but also a fusible silicate or glass component. Plasma or flame spray coatings utilise the partial or full melting of a powder as it passes through a high temperature gas plasma induced electrically or by a combustion gas flame. viOne of the oldest and simplest coating technologies is electroplating of metals. Sol-gel coatings and coatings based on the pyrolysis of metal-organic films are a technology of increasing significance. Chemical vapour deposition (CVD) uses vapour phase transport to grow epitaxial and highly structured films on crystal substrates. One of the exciting recent coating technologies is that of the formation of diamond films by the decomposition of organic-metallic compounds such as methane in an electrical plasma. No matter which kind of methods is used, layers of metals, ceramics or composites having thickness from angstrom level to the few hundered micrones are important part of industrial practice. Ceramic coating technologies have been showing continuous progress. In U.S.A., the amount of ceramic coatings in 1990 has $ 371 million value, which will have export value in the year 2000 $ 940 million. According to these to concentrate on ceramic coatings has not only scientific and technological importance, also high amount of economical value. In this study, boronizing practice was intended to use for surface modification of steels. Boronizing is frequently used to increase surface hardness to obtain a layer resisting wear and chemical attacks. Boronizing being a thermo-dif fusional process being preferred because of several reasons; some of them are: (i) The advantage of raw material; Turkey is not dependent on other countries in boron minerals (economical reason). (ii) Easy of method (practical reason). (iii) Lack of the knowledge on the alloy effect. (iv) Comparison of commercial and impurity level controlled alloy after boronizing. (v) To find out a way to produce a surface with good mechanical properties, such as hardness, toughness, tribological properties and chemical properties etc. Three commercial alloys, AISI 1020, AISI 1040, AISI 1050 and 6 special alloys prepared by using poor elements in order to control the level of impurity and alloying elements. Technological efforts to improve boriding commercial alloys have been pursued by many researchers. Little attention was paid to the special alloys to study individual role of each alloying element. It is well known, that thermochemical boriding of iron alloys is dependent on the matrix chemical composition among the other variables. viiAlloying elements in base metal play important role on the nucleation of borides, diffusion characteristics, relative amount of reaction products, modification of composition and boride layers, shapes of grown surface layers. Chemical composition of 6 special alloys used are shown in Table 1. Impurity level was kept constant in all special alloys. Since each specially prepared steel has either Mn or Cr or Mn+Cr or no alloying element to compare the effect of alloying element separately and together, the role of Mn and Cr content was outlined. Boronizing bath consisted of 66% Borax, 14% Boric Acid and 20 % Ferro-Silicon. All 9 alloys were boronized in the same condition, at 940°C. Boronizing period was changed from 1/2 hrs to 10 hrs., so that the effect of time was studied. After boronizing, each group of alloy was examined by using classical metallographic techniques, X-ray diffraction, hardness measurements, fracture toughness measurements, Scanning Electron Microscopy and Corrosion Tests. Metallographic studies showed that coating layer has columnar shape. The length and interface between adjacent columns depends on alloy and boronizing time. Initially, the thickness of boronized layer increases, later on, the rate of increase decreases. It is well: understood, that procces is diffusional. The hardness level in the layer ' also depends on alloy and boronizing time. By hardness measurement from the surface until the original matrix, coated material are divided into three parts; vxn(i) Layer having borides, (ii) The region being below column, where boron makes solid solution has hardness less than that of ".column and higher than that of original alloy, (iii) Matrix, which are not affected by boron. It is also found, that hardness doesn't depend on the hardness of original non-coated alloy. ' Hardness is essentially related with boron compounds, which layers have. X-ray diffraction analysis showed, that layer has ceramic composites in each alloy. All alloys have at least two types of iron borides; FeB and Fe2B.- Carbon steels and special alloy without alloying element have only two types of iron borides. However. it should be noted that special steel without alloying element had greater hardness on the surface than those of commercial alloys, although both commercial alloys and special steel without alloying element had only iron borides on their surfaces. The other five alloys with Mn or Cr or Mn+Cr have in addition to iron borides» manganese or/ and chromium borides depending on boronizing time. Alloy with 0.5% Mn after 5 and 7 hrs boronizing time had MnB on surface in addition to FeB and Fe2B. Whereas alloy with 1% Mn after 5 hrs. had only Mn2B, after 7 hrs. both MnB and Mn2B in addition- FeB and Fe2B. For chromium, situationwaa.the sameexcept alloy with 0.5% and 1% Cr had both Cr2B and CrB after 7 hrs. Alloy with both Mn and Cr had two types of iron, manganese and chromium borides. X-ray diffraction did not only reveal the types of borides, but showed also there is orientation differences for every borides, which may affect the directional properties. ' Through SEM analysis, the change in chemical composition was studied. In each column, itwas found, that the amount of alloying element fluctuates relatively. Combination of X-ray diffraction studies with SEM results on the fluctuation of chemical analysis shows, that strong inhomogeneities exist structurally and chemically, although SEM micrographs distinguish by the help of contrast only two different types of borides, e.g. Fe2B and FeB. Although the value of fracture toughness was changing around 1 MPa m^2, it was noticed that Mn has only positive role on the fracture toughness. IXHowever it must be noted that types of borides on the surface are at least more than one. Each one, individually, has its own fracture toughness value, but it is not known which kind of interactions occurs among borides in order to influence on fracture value of borides composites being ceramic composites. On the other hand directionality or texture, which is found by X-ray diffraction, may be effective. Excluding the effect of texture, it is concluded taht preparation of independent intermetallic borides would be helpful to distinguish the influence of single type of boride, but in this case due to preparation methods of intermetallics, borides produced would not be same borides except chemical composition. In the severe condition all boride layers showed good corrosion resistance. Certainly boronizing increases the corrosion resistance. However, the activities of interfaces between, (i) Columns, (ii) Different borides, (iii) Borided surface and interface in corrosive environment under stress or without stress in order to understand how crack nucleates and propagates, are not known yet. x