Compressor Instability with Integral Methods Episode 2 Part 2 potx

Chapter 5 Substrate and Coating Erosion 5.1 Introduction The basic process of blast cleaning is the impingement of individual abrasive par- ticles under different conditions on the target material. In the reference literature, this process is often referred to as solid particle erosion. The system “coating – interfacial layer – substrate” is illustrated in Fig. 5.1. This complete system responds by the following two mechanisms to the impingement of solid particles: r erosion of the coating material (cohesive mode); r debonding of the coating material (adhesive mode). These mechanisms are illustrated in Fig. 5.2, and they will be discussed in detail in the subsequent sections. Erosion occurs usually either if the coating is rather thick or if the adhesion of the coating to the substrate is very good. The erosive response of bulk (coating) ma- terials can be subdivided into elastic response (brittle) and elastic–plastic response (ductile). These modes of response are illustrated in Fig. 5.3 in terms of scratching images of organic coatings. Debonding and delamination in the interface between substrate and coating are alternative coating removal mechanisms, and they occur at rather small coating thickness, or if the adhesion is low. 5.2 Mechanical Properties of Oxides and Organic Coatings 5.2.1 Relevant Mechanical Properties The material to be considered for blast cleaning is rather a material system com- posed of the three following parts: r substrate; r interfacial layer; r layer (coating, oxide). A. Momber, Blast Cleaning Technology 167 C  Springer 2008 168 5 Substrate and Coating Erosion Fig. 5.1 System: substrate (aluminium) – interface – coating (epoxy). Photographs: Zhang et al (2003). Left: deteriorated adhesion: Right: good adhesion (a) (b) Fig. 5.2 Basic types of coating response to solid particle impingement (Strojny et al., 2000). (a) Buckling and debonding; (b) Brittle response with bulk material erosion Fig. 5.3 Basic types of organic coating response to scratching (Randall, 2006). (a) PVC-based hardcoat finish: elastic response with cracking; (b) Silicone finish: elastic-plastic response with permanent deformations; (c) Automotive varnish coat: plastic response with some ruptures 5.2 Mechanical Properties of Oxides and Organic Coatings 169 Such a system is shown in Fig. 5.1. Whether cohesive mode or adhesive mode dominates depends on the adhesion between coating and substrate and on coating thickness. If both adhesion strength and coating thickness are low, adhesive delam- ination occurs. For the cohesive mode, the bulk properties are of importance, but it is known that, for coating materials, cohesive properties, e.g. indentation properties, depend on the distance to the substrate (Roche et al., 2003). Over the years, many erosion studies have been performed on a variation of ma- terials. It was shown that no single material property can determine the resistance of a material against the erosion by impinging solid particles. However, there are some properties which were observed to notably affect the erosion resistance of materials. These cohesive material properties include the following: r hardness; r Young’s modulus; r strain energy density; r tensile strength; r fracture mechanics parameters. Adhesive system properties, say adhesion strength between substrate and adher- ing layer (oxide, glue and coating), also affect the behaviour of the system. An extensive review about parameters and measurements methods for coating materials is provided by Papini and Spelt (2002). The cohesive properties can be exhibited in a stress–strain diagram of a stressed material volume. Typical stress–strain diagrams for three types of material response are shown in Fig. 5.4. The plot in Fig. 5.4a illustrates the linear-elastic response of a material. This response is characterised by the damage features shown in Fig. 5.3a. The plot in Fig. 5.4b illustrates the elastic–plastic response of a material. This re- sponse relates to the damage features shown in Fig. 5.3b. The shapes of stress–strain curves are not general material properties, but depend on the loading conditions. Stress–strain curves of paint materials are, for example, sensitive to the loading rate. This aspect was in detail investigated by Dioh and Williams (1994). The Hardness and Young’s modulus represent the deformation response of a material. Hardness is stress strain E SD σ T σ T σ E σ Y ε ε strain toughness strain stress fracture stress stress ε T (a) (b) (c) Fig. 5.4 Stress–strain diagrams and deformation parameters. (a) Linear-elastic response; (b) Elastic-plastic response; (c) Dynamic compressive diagram (Levin et al., 1999) 170 5 Substrate and Coating Erosion for many materials coupled with the yield strength (Tabor, 1951), and for elastomers it is linearly related to Young’s modulus (Li and Hutchings, 1990). Strain energy density is the area under the stress–strain curve of a material: E SD =  ε T 0 σ(ε)dε (5.1) It has the unit kJ/m 3 which is that of a specific volumetric energy. For elastically responding materials, elastic strain energy density can be approximated as follows: E SD = σ 2 T 2 · Y M (5.2) The variables are illustrated in Fig. 5.4a. The elastic strain energydensity has been applied by several authors as a parameter which can characterise the resistance of a material against solid particle erosion (Bitter, 1963; Kriegel, 1968). The detailed shape of a stress–strain curve can already deliveraroughestimate of the behaviour of the material in question. Some examples are provided in Fig. 5.5. The term “hard” in this graph corresponds to elastic response, whereas the term “soft” points to a rather plastic material response. Levin et al. (1999) extended the stress–strain approach and developed an idea to apply a dynamic (high strain rate) compression stress– Fig. 5.5 Stress–strain diagrams for different response characteristics (Hare, 1996). Material char- acteristics: 1 – hard and brittle; 2 – hard and strong; 3 – hard and tough; 4 – weak and brittle; 5 – soft and tough; 6 – soft and weak 5.2 Mechanical Properties of Oxides and Organic Coatings 171 strain curve for the assessment of the response of ductile metals to solid particle erosion. This approach is illustrated in Fig. 5.4c. The area under the modified stress– strain curve, denoted “tensile toughness” by the authors, characterises the energy absorbed during theerosion process. The failure stressestimated from sucha dynamic stress–strain curve is related to the hardness of the eroded surface as follows: σ E = A D · H M (5.3) The constant had values between A D = 3.4 and 4.0 for many metal alloys (Levin et al., 1999). Mechanical properties depend on a number of physical effects, namely strain rate sensitivity and temperature sensitivity. Solid particle impingement, the basic mech- anism for blast cleaning, is associated with high strain rates (see Sect. 5.4.1). Values as high as 10 6 per second can be assumed (Hutchings, 1977a). The response of metals and organic coatings to loading depends on deformation velocity and strain rate. An increase in strain rate may, for example, change the response of organic coatings from plastic deformation to intense chipping, and may cause a two-fold increase in yield stress (Dioh and Williams, 1994). An increase in deformation rate rises tensile strength of organic lacquers (Skowronnek et al., 1991). An increase in strain rate also modifies material properties as shown in Fig. 5.6 for the yield stresses of organic materials (see Fig. 5.4b for the definition of the yield stress). Siviour et al. (2005) found for polymer materials a change in the structures of stress–strain curves if temperature and strain rate were varied. Fig. 5.6 Strain rate effects on yield strength of polymeric materials (Kukoreka and Hutchings, 1984). Materials: 1- polyethersulphone; 2- polycarbonate; 3- high-density polyethylene 172 5 Substrate and Coating Erosion Table 5.1 Dynamic hardness values of steel blades and deposits (Raykowski et al., 2001) Material Dynamic hardness in GPa Compressor blade deposit 0.22–0.26 Compressor substrate 0.3–0.43 Turbine blade deposit 6.3 Based on a balance between the kinetic energy of an impinging particle and the work done in plastically deforming the impinged coating material, Tangestanian et al. (2001) derived a dynamic hardness: H d = m P · v 2 P 2 · π ·   d 2 P − a 2 Cmax  1/2 ·  1 3 · a 2 Cmax + 2 3 · d 2 P  − 2 3 · d 3 P  (5.4) This parameter is defined as the instantaneous force resisting indentation during a collision divided by the instantaneous contact area. The dynamic hardness is an important property in determining the impact behaviour at high strain rates. Values for this material parameter are listed in Table 5.1. Temperature variations can be responsible for ductile–brittle (plastic–elastic) transition of coatings under impact conditions (Moore, 2001). At low temperature, brittle fracture will occur with a comparatively low amount of absorbed impact en- ergy. A ductile–brittle transition will occur at some fixed temperature. Other coating properties, namely deformation properties and fracture properties, also depend on temperature; examples are provided in Fig. 5.7 for the variations in fracture tough- ness and yield strength. Figure 5.8 illustrates the general effect of temperature vari- ations on the behaviour of organic materials. Fracture mechanics parameters include mainly fracture toughness and energy release rate. Both parameters can be applied to individual materials, but also to Fig. 5.7 Effect of temperature on mechanical properties of organic coating materials (Moore, 2001). Left: effect on fracture toughness; right: effect on yield strength 5.2 Mechanical Properties of Oxides and Organic Coatings 173 Fig. 5.8 Effects of temperature on the behaviour of organic coating materials (Zorll, 1984) interfaces between two materials, say coating and substrate, as well as to joints. Principles of fracture mechanics with respect to contact mechanics and erosion are described in Lawn’s (1993) book. The fracture toughness characterises a critical value for the stress intensity at the tip of a crack required to extend the crack. It is defined as follows: K Ic = α C · σ T · (π ·l C ) 1/2 (5.5) In that equation, l C is the crack length, σ T is the failure tensile stress and α C is a shape factor. The fracture toughness must be estimated experimentally. Its physical unit is MN/m 3/2 . The critical energy release rate is defined as follows: G Ic = K 2 Ic Y M (5.6) The critical energy release rate characterises the specific energy required to extend a crack. Its physical unit is J/m 2 . The subscript “I” in (5.5) and (5.6) shows that both relationships are valid for a tensile loading mode (mode I) only. A method for the estimation of critical energy release rate for the interfacial zone between steel substrates and adhesives under impact load was developed by Faidi et al. (1990). A typical value for combination steel-epoxy was G Ic =0.15kJ/m 2 . State-of-the- art measurement methods for the assessment of fracture mechanics parameters for organic coatings as well as for interfaces between organic coatings and substrate materials are described in detail by Papini and Spelt (2002). 174 5 Substrate and Coating Erosion 5.2.2 Mechanical Properties of Oxides 5.2.2.1 Deformation Parameters Oxides are basically formed either due to atmospheric effects (corrosion) or due to thermal effects (mill scale). The composition of oxides is complex, and they usu- ally consist of numerous layers with different chemical compositions. Detailed de- scriptions of mill scale compositions are provided by Wirtz (1962). The mechanical properties of mill scale depend mainly on the formation temperature. The effect of temperature on the Young’s modulus of growing mill scale layers was investigated for different metals by Hurst and Hancock (1972) and Tangirala (1998). For high temperatures, Young’s modulus reduced. Typical values for Young’s modulus of scales were: Y M =2× 10 5 MPa for iron, Y M =3× 10 5 MPa for nickel, and Y M = 2.2 × 10 5 MPa for an alloyed steel. Table 5.2 lists further elastic parameters for iron oxides at different formation temperatures. For comparison, the elastic parameters of the plain iron are also listed in the table. 5.2.2.2 Hardness Results of microhardness measurements on oxides of numerous metals were re- ported by Lepand (1963),Wood and Hodgkiess (1972) and Zieler and Lepand (1964); some results are listed in Table 5.3. It was found that microhardness can basically be related to the crystal structure of the oxides. Oxides with a rhombohedral struc- ture (e.g. Cr 2 O 3 ) featured very high hardness values. When layered structures were formed on pure metals, e.g. FeO, Fe 3 O 4 and Fe 2 O 3 on iron, the hardness increased from the metal towards the oxide. 5.2.2.3 Adhesion Parameters Spangenberg (1972) and Engell (1960) performed investigations into the adhesion strength of mill scale to metal substrates. Spangenberg (1972) utilised three steel types as listed in Table 5.4. He derived an empirical relation of the general form: σ M = C 1 + C 2 · h Z + C 3 · ϑ R + C 4 · ε M (5.7) Table 5.2 Mechanical properties of iron and iron oxide (Tangirala, 1998) Parameter Material Formation temperature in ◦ C 570 674 743 800 Young’s modulus in GPa Iron oxide 182 168 158 151 Iron 155 150 135 125 Poisson’s ratio Iron oxide 0.34 0.34 0.34 0.34 Iron 0.34 0.34 0.34 0.34 Fracture stress in MPa Iron oxide 38 2.4 1.9 4.9 5.2 Mechanical Properties of Oxides and Organic Coatings 175 Table 5.3 Microhardness values of metal oxides (Zieler and Lepand, 1964; Wood and Hodgkiess, 1972) Oxide Hardness in kg/mm 2 ZnO 184 NiO 600 TiO 2 624 Cu 2 O 232 Cr 2 O 3 1,820–3,270 FeO 270–390 Fe 2 O 3 690 α−Fe 2 O 3 986–1,219 Fe 3 O 4 420–500 α−Al 2 O 3 2,160 Here, σ M is the adhesion strength of the mill scale to the substrate (N/cm 2 ), h Z is the mill scale layer thickness (μm), ϑ R is the rolling temperature ( ◦ C) and ε D is the deformation degree (%). The deformation degree is a function of the steel plate thickness before and after the rolling process. Typical values for the adhesion strength as well as the constants C 1 to C 4 are listed in Table 5.4. The effect of the mill scale layer thickness is most important. The effect of the oxidation temperature was investigated in more detail by Engell (1960). This author found that adhesion of oxides to iron is best at moderate temperatures; an example is provided in Fig. 5.9. 5.2.3 Mechanical Properties of Organic Coatings 5.2.3.1 Deformation Parameters Paul et al. (2004) have shown that numerous organic coating materials (e.g. oxide primer, polyurethane-based enamel) feature a linear stress–strain behaviour accord- ing to Fig. 5.4a. The progress of the stress–strain function, thus Young’s modulus, depended on coating composition. Figure 5.10 shows the effects of hardener con- centration and film thickness on the Young’s modulus of organic coatings. It can be noted in the figure that coating dry film thickness affects the mechanical parameter; the higher the film thickness, the higher the values for Young’s modulus. Values for Table 5.4 Adhesion strength values for mill scale (Spangenberg, 1972) Parameter σ M in N/cm 2 Steel type Armco iron St 42 St 70 200–1,600 100–1,800 200–1,600 C 1 531 1,649 2,019 C 2 11.2 3.5 5.4 C 3 −0.8 −1.8 −1.8 C 4 5.6 9.0 0 176 5 Substrate and Coating Erosion Fig. 5.9 Effect of oxidation temperature on the adhesion of oxides to the metal substrate (Engell, 1960) Fig. 5.10 Effects of hardener concentration and coating thickness on Young’s modulus (Fokke, 1999) [...]... kJ/m2 Tensile modulus in MPa R1 R2 F1 F2 L1 L2 29 .6 24 .4 17.6 17.5 26 .8 51.6 2. 1 1.7 22 .8 15.7 30.7 4.7 8.5 5.5 69.9 50.0 161.0 39.7 1,740 1,680 29 7 423 1,160 1,870 178 5 Substrate and Coating Erosion Table 5.6 Mechanical data for polymeric coatings (Trezona et al., 1997) Coating F1 F2 F3 F4 R1 R2 R3 R4 R5 R6 L1 H1 Polymer type Acrylic Acrylic Acrylic 2C PU Acrylic Acrylic Acrylic Acrylic 2C PU 2C PU 2C... Acrylic Acrylic 2C PU 2C PU 2C PU 2C PU Peak stress in MPa Failure strain in % Tensile modulus in MPa Tensile failure energy in MJ/m3 17.6 25 .8 49.1 57.7 24 .4 29 .6 47.1 77.0 77.0 61.6 26 .8 51.6 22 .8 35.1 14.3 29 .0 1.7 2. 1 2. 9 4.1 5.9 8.4 30.7 4.7 29 7 855 1,375 1,418 1,680 1,740 1,8 02 2,617 1,990 1,614 1,160 1,870 3.04 5 .25 5.10 9.19 0.17 0 .24 0.65 1.87 2. 27 3.60 4. 02 0.87 Table 5.7 Brinell hardness... 0.11 0.37 1 92 5 Substrate and Coating Erosion Table 5.13 Friction values for particle impingement situations (Yabuki and Matsumura, 1999) Particle Particle diameter in μm Particle velocity in m/s 3,000 3,000 600 Brass shot 3,000 3,000 3,000 Steel grit 28 0 880 1,130 Silicone carbide 27 0 29 0 560 Silica sand 26 0 340 400 Friction coefficient 0.8 0.9 1.4 1.0 1 .2 1.5 3.0 2. 1 1.7 2. 7 2. 3 2. 3 2. 5 2. 3 2. 1 Steel... 0 .22 0.19 0.18 0 .26 0 .22 0.17 0.36 0 .28 0 .23 0.39 0.39 0.38 0 .29 0 .26 0 .25 5.4 Material Loading Due to Solid Particle Impingement 5.4.1 Loading Parameters Tensile stresses generated in an elastically responding material by an impinging spherical particle have maximum values at the surface at the edge of contact according to Hertz’s (18 82) theory for elastic contact: σT = (1 − 2 · νM ) · FC 2 · π · a2... (Gnyp et al., 20 04) Coating name Thickness in Treatment Brinell and composition μm temperature hardness in in ◦ C MPa KO-FMI-5 400–450 Laquer + coal ash + close packing with ultrasound Pompur 804 Ambercoat 20 00 Steel substrate 400–450 20 150 170 25 0 20 303 27 7 509 391 25 8 – – – 150 170 25 0 – – – 303 407 375 143 21 3 1,310 Table 5.8 Vickers hardness values for organic coatings (Rehacek, 19 82) Alkyd resin... P π · 2 mP 2 · π · rP · HID 1 /2 (5 .28 ) The plastic contact time mainly depends on the dynamic hardness of the target material, and it is independent of the impact velocity The total contact time is given by: tP = tE + tPL P P (5 .29 ) An approximation for the strain rates associated with high-speed particle impingement was provided by Hutchings (1977) who derived the following equation: 1 /2 23 /2 vP ˙... 5.4 .2 Material Response to Particle Impingement Depending on the contact situation, materials respond either elastic or plastic to solid particle impingement Examples are shown in Fig 5 .21 The critical particle velocity for plastic flow during particle impact is (Johnson, 1985): v2 = PL 26 · (σf /YM )4 · σf ρP (5.31) The threshold particle velocity for Hertzian crack formation can be derived from (5 .24 )... coatings (Rehacek, 19 82) Alkyd resin Vickers hardness Amount of in MPa plastic deformation in % Alkyd Alkyd with low soy bean oil content Alkyd with moderate linseed oil content Alkyd with moderate soy bean oil content Alkyd with high soy bean oil content 82 66 58 17 5.7 34 ± 4 47 ± 5 42 ± 1 53 ± 2 25 ± 8 5 .2 Mechanical Properties of Oxides and Organic Coatings 179 Table 5.9 Results of deformation measurements... (Knight et al (1977): 2 · π · ρP · r3 2 P · vP = 3 z max FC (z) dz 0 (5 .24 ) 5.4 Material Loading Due to Solid Particle Impingement 193 A solution to (5 .24 ) is: FC = 5 · π · ρP 3 3/5 · 2/ 5 3 · kE 4 6/5 · vP · dP 2 2 (5 .25 ) 6/5 [The relationship FC ∝ vP was already found by Hertz (18 82) .] The parameter kE balances the elastic properties of particle and target material according to (5.14) A combination of... 5 .21 Types of response to solid particle impingement (Aquano and Fontani, 20 01) (a) Elastic response with cone crack formation; (b) Plastic–elastic response at different particle impact velocities (ϕ = 25 ◦ ); the lower drawing is adapted from Winter and Hutchings (1974) 196 5 Substrate and Coating Erosion Fig 5 .22 Impact transition criterion for coating materials according to (5.33) dPL ∝ H−3 M 2 . and Hodgkiess, 19 72) Oxide Hardness in kg/mm 2 ZnO 184 NiO 600 TiO 2 624 Cu 2 O 23 2 Cr 2 O 3 1, 820 –3 ,27 0 FeO 27 0–390 Fe 2 O 3 690 α−Fe 2 O 3 986–1 ,21 9 Fe 3 O 4 420 –500 α−Al 2 O 3 2, 160 Here, σ M is. 17.6 22 .8 29 7 3.04 F2 Acrylic 25 .8 35.1 855 5 .25 F3 Acrylic 49.1 14.3 1,375 5.10 F4 2C PU 57.7 29 .0 1,418 9.19 R1 Acrylic 24 .4 1.7 1,680 0.17 R2 Acrylic 29 .6 2. 1 1,740 0 .24 R3 Acrylic 47.1 2. 9. % Energy to break in kJ/m 2 Tensile modulus in MPa R1 29 .6 2. 1 8.5 1,740 R2 24 .4 1.7 5.5 1,680 F1 17.6 22 .8 69.9 29 7 F2 17.5 15.7 50.0 423 L1 26 .8 30.7 161.0 1,160 L2 51.6 4.7 39.7 1,870 178