**5.6.2 Refractory alloy**

To this group belong the high temperature resistant metallic materials. They are used in gas turbines manufacturing, reaction engines, inner parts of furnaces. They must present a good corrosion resistance at high temperature (the oxidation phenomena at high temperature) and be capable to support attacks from combustion ashes (in particular S, V and its derivates, contained in the ashes), and good creep strength (creep-fluage). They are distinguished in:


Low C tenor alloy (except some martensitic steels), or better Fe-Cr or Fe-Cr-Ni alloy, with Cr percentage varying from the 12 to 30% and of Ni from the 0 to 35% (main characteristic is the corrosion resistance). Such characteristic is due to the passivation property that such steels, in oxidation conditions, have (that is capacity to create an oxide film of extremely thin dimensions whose characteristics remarkably according to change the chemical composition of the alloy, of the heat treatment, the structural composition, the superficial stress state) .

The martensitic steels have the critical points and are submitted to heat treatment; they have higher mechanical characteristics in comparison to the ferritic and austenitic ones, but lower corrosion resistance. The ferritic steels, instead, do not possess the critical points, can only be submitted to re crystallization annealing treatments; they have good resistance to oxidation at high temperature, as well as more elevated how much greater are the chromium content. the austenitic steel are, between the stainless steel, those mainly produced. They have elevated corrosion resistance in numerous atmospheres; they are submitted to heat volatilization treatment (hardening of austenitic steel) to melt, in the austenitic matrix, the Cr carbides (their presence decrease the corrosion material resistance). For this same reason the C tenor has to be held to low values. Besides these three classes are the stainless steel hardening by precipitation, with high mechanical characteristics, whose corrosion resistance, also being lower than the austenitic types, is higher than martensitic ones. Hardening process derives by the precipitation insoluble phases dispersed during the aging

To this group belong the high temperature resistant metallic materials. They are used in gas turbines manufacturing, reaction engines, inner parts of furnaces. They must present a good corrosion resistance at high temperature (the oxidation phenomena at high temperature) and be capable to support attacks from combustion ashes (in particular S, V and its derivates, contained in the ashes), and good creep strength (creep-fluage). They are

Normally they are subdivided in following the three classes:

process. Between this type of steels we remember:

2. high tensile steels (UNI 4010); 3. usury resistant steels (UNI 3160); 4. heat resistant steels (UNI 3159); 5. corrosion resistant steels (UNI 3161); 6. creep resistant steels (UNI 3608).

**5.6.2 Refractory alloy** 

distinguished in: • Fe base alloy; • Ni base alloy; • Co base alloy.

1. quality steels for mechanical constructions (UNI 3158);

**5.6 Metallic materials** 

• Martensitic steel ; • Ferrite steel; • Austenitic steel.

**5.6.1 Stainless steel alloy** 

The refractory steels are used for those products whose main characteristic is the resistance to hot gases and residual of combustion, at higher temperature than 550 °C. In European code for these materials is EURONORM 95.

### **5.6.3 NICHEL alloy (Ni)**

Melting point is approximately 1455 °C. The Ni, in steels alloy, reduces the grain dimensions, increases the depth of hardening and renders them less sensitive to the low temperatures fragile failure. The Ni can be bond with numerous metals (particularly with Fe, Cr, Cu). The Ni- Fe alloys can be:


### **5.6.4 COBALT alloy (Co)**

It is used in metallurgy with Fe, Ni, Mo, W and other metals for the production of refractory alloys, steels for tools and alloys for permanent magnets. It is a constituent of maraging steels.

## **5.6.5 TITANIUM and TITANIUM alloy (Ti)**

Ti is recently used in the industrial manufacturing but its utilization still is limited, due to the high fabrication cost. The main properties of this metal and its alloys are the oxidation resistance also to 400-450 °C and higher mechanical characteristics both cold and hot environment. Its low density (4,54), has as consequence an high σm/p.sp ratio. The melting point is 1672 °C. These characteristics of pure Ti and its alloys are a lot influenced by the inner contained gas and, in particular, by the H. The value does not have to exceed the 0,0125% to avoid a dangerous embrittlement. The Ti alloys find application in the fabrication of supersonic vehicles, the aerospace constructions and in particular in the missiles.

### **6. Manufacturing technologies and process**

The UMGT design is strongly limited by the available technologies. The micro manufacturing introduces a several problems in the development of every single part of the device. All studies have bring to adopt as manufacturing techniques the DRIE (Deep Reactive Ion Etching) and the Wafer Bonding, diffuse and by now widely mature, add to Micro Reaction Sintering, a new process based on the HIP (Hot Isostatic Pressing), already experimented for the fabrication of micro turbines and capable to realize machines with an accuracy of the order of the μm, in small series and with contained production costs.

Ultra Micro Gas Turbines 41

geometry is obtained. Several materials and techniques exist to produce the photo resistant mask, and SiO2 is the most diffuse used oxide. The etching plasma beam can be composed by ion mixtures of varied nature. The process is described in Figure 27. Numerous manufacturing companies have developed particular and sophisticated machine capable to deeply etching with extreme precision, even if the adopted process is usually the Bosch patented one. Moreover several machines and device commercialized from the Alcatel, are capable to achieve an etching speed of 20 μm/min with aspect ratio of 60:1, but with the possibility to arrive till 100:1 with an ulterior cost increase. Practically, without excessive expenses, the stamps for the turbine in less than an hour and for the compressor in about 30 min could be recorded, without the necessity of successive manufacturing, since this technique has an accuracy of the order of the μm. Add the possibility to obtain tapered shovels and etchings of about 0,25 μm. The disadvantage of this technique is that simply extruded geometries can be only obtained, but already is being worked to create masks in greys tonality to producing more complex forms and shape, that could allow to have a height of the channels variable,

This technique allows to create complex structures joining, using different other materials which the glass, wafers of already micro etched silicon. The fusion between the several parts is superficially obtained by cooking the parts, in presence of a strong cross-sectional electrostatic field that crosses the pieces interface. This system creates an alloy; the high temperatures strengthen the bonds (anodic fusion). In alternative, it is possible to use a melting process (direct fusion) in presence of modest pressure with heat transfer or it can use adhesive materials that joins the parts through superficial chemical reactions. The first two techniques create strong covalent type bonds, but they have the disadvantage to demand extremely smooth and clean surfaces for being effective, as the first step for a good resolution of the fusion is to bond the surfaces by the Van der Waals intermolecular forces. The product quality is correlated to the process temperature, between the 800°C and the 1200°C, levels of equal resistance than the crystalline structure of the same material are achieved, but already between 200°C and the 400°C sufficiently strong bonds are realized by the chemical activation of surface reactions. To improve the fusion a solution obtained from the combination of the several procedures can be used. The alignment of the layers, thanks to laser techniques, reaches precisions of the order of the μm, even if, currently, it is not

This manufacturing techniques is developed for the production of SiC turbomachines rotors of 5÷10 millimetre of diameter. From graphite powders, α-SiC, silicon phenol resin is inserted in a stamp, derived from silicon wafer. The parts of the stamp are melted and all component is submitted to high temperature HIP. The technique has been calibrated in several conditions of pressure, temperature and composition of base powders and it has been verified that the microcrystalline structure of the final piece has an higher quality if the furnace temperatures is about 1500-1700°C, the pressure varies from 100 to 50 MPa and using α-SiC powders instead of silicon one. A microscopic analysis of the structure has demonstrated that melted stamp reacting with the graphite substrate forms a covering of β-SiC around a nucleus of α-SiC deriving from initial powders. But is not still available

with the consequent possibility to adopt thin profiles for the turbo machines.

possible to align more than 5-6 wafer maintaining such accuracy.

**6.3 Micro Reaction Sintering (MRS)** 

**6.2 Wafer bonding** 

### **6.1 DRIE (Deep Reactive Ion Etching)**

This technique essentially consists in etching wafer of base material, usually silicon microcrystalline, through a plasma beam. Considering that a wafer has a diameter between 100 and 300 millimetre, it is possible, at the same time to produces numerous devices, in parallel, as shown in Figure 26.

Fig. 26. Series of nano devices on a single wafer [Epstein 1997]

Initially the wafer is covered by an oxide and by a photo resistant film, on which a so-called "mask" is transferred through optical methods, a model in black and white with the geometry to recording. The transfer is realized bombing with ultraviolet rays a glass plate to contact directed with the wafer, on which the mask is applied. Geometry is so "developed" on the photo resistant material as if it were a normal photographic film. Finally and the piece is "cooked". After the "development", some oxide exposed parts remain, that are removed using solvents, leaving some area covered by the mask and some zones uncovered by silicon material. The piece is, then, etched, but the speed of the oxide etching is 50-100 times lower than of the silicon one. At the end of the procedure, the extruded initial

Fig. 27. DRIE manufacturing process phases

This technique essentially consists in etching wafer of base material, usually silicon microcrystalline, through a plasma beam. Considering that a wafer has a diameter between 100 and 300 millimetre, it is possible, at the same time to produces numerous devices, in

Initially the wafer is covered by an oxide and by a photo resistant film, on which a so-called "mask" is transferred through optical methods, a model in black and white with the geometry to recording. The transfer is realized bombing with ultraviolet rays a glass plate to contact directed with the wafer, on which the mask is applied. Geometry is so "developed" on the photo resistant material as if it were a normal photographic film. Finally and the piece is "cooked". After the "development", some oxide exposed parts remain, that are removed using solvents, leaving some area covered by the mask and some zones uncovered by silicon material. The piece is, then, etched, but the speed of the oxide etching is 50-100 times lower than of the silicon one. At the end of the procedure, the extruded initial

**6.1 DRIE (Deep Reactive Ion Etching)** 

Fig. 26. Series of nano devices on a single wafer [Epstein 1997]

Fig. 27. DRIE manufacturing process phases

parallel, as shown in Figure 26.

geometry is obtained. Several materials and techniques exist to produce the photo resistant mask, and SiO2 is the most diffuse used oxide. The etching plasma beam can be composed by ion mixtures of varied nature. The process is described in Figure 27. Numerous manufacturing companies have developed particular and sophisticated machine capable to deeply etching with extreme precision, even if the adopted process is usually the Bosch patented one. Moreover several machines and device commercialized from the Alcatel, are capable to achieve an etching speed of 20 μm/min with aspect ratio of 60:1, but with the possibility to arrive till 100:1 with an ulterior cost increase. Practically, without excessive expenses, the stamps for the turbine in less than an hour and for the compressor in about 30 min could be recorded, without the necessity of successive manufacturing, since this technique has an accuracy of the order of the μm. Add the possibility to obtain tapered shovels and etchings of about 0,25 μm. The disadvantage of this technique is that simply extruded geometries can be only obtained, but already is being worked to create masks in greys tonality to producing more complex forms and shape, that could allow to have a height of the channels variable, with the consequent possibility to adopt thin profiles for the turbo machines.

## **6.2 Wafer bonding**

This technique allows to create complex structures joining, using different other materials which the glass, wafers of already micro etched silicon. The fusion between the several parts is superficially obtained by cooking the parts, in presence of a strong cross-sectional electrostatic field that crosses the pieces interface. This system creates an alloy; the high temperatures strengthen the bonds (anodic fusion). In alternative, it is possible to use a melting process (direct fusion) in presence of modest pressure with heat transfer or it can use adhesive materials that joins the parts through superficial chemical reactions. The first two techniques create strong covalent type bonds, but they have the disadvantage to demand extremely smooth and clean surfaces for being effective, as the first step for a good resolution of the fusion is to bond the surfaces by the Van der Waals intermolecular forces. The product quality is correlated to the process temperature, between the 800°C and the 1200°C, levels of equal resistance than the crystalline structure of the same material are achieved, but already between 200°C and the 400°C sufficiently strong bonds are realized by the chemical activation of surface reactions. To improve the fusion a solution obtained from the combination of the several procedures can be used. The alignment of the layers, thanks to laser techniques, reaches precisions of the order of the μm, even if, currently, it is not possible to align more than 5-6 wafer maintaining such accuracy.
