Surface Characterization after Blasting

Abstract

Blasting modifies the surface state of materials in terms of surface irregularities too. Bearing in mind that the roughness characteristics affect the components functionality, it is essential to study and evaluation the surface state of pretreated materials. The chapter deals with evaluation of relation between individual surface roughness parameters of the blasted surfaces based on the measured values on the surfaces, which were blasted by various types of blasting materials. Based on the analysis of the results were also proposed sets of surface roughness parameters, which can be used in the assessment of the blasted surfaces. These allow you to effectively distinguish differences in roughness of blasted surfaces from the point of view of other follow-up technologies. It also lists the main factors that affect surface roughness.

1. Introduction

The blasting process involves several simultaneously occurring processes. From the point of view of blasting effect on the base material, it is necessary to understand blasting process as the process of surface deformation or elastic-plastic deformation of metal in its full volume.

The quality of blasted surface is characterized by the following features, namely:

1. The geometrical shape of the blasted surface, which involves the micro-geometry, a surface volume, and the real size of the surface;

2. The condition of the blasted surface, which involves a plastic deformation, strengthening, a thermal effect, structural changes, changes of mechanical and technological properties, and the residual stresses;

3. The cleanliness of the blasted surface.

From the point of view of a component function, geometrical properties of a surface are very important in many cases (e.g., in conditions of friction and wear). Therefore it is necessary to put a sufficient emphasis on evaluation of surfaces geometry and the determination of profile deviations from the defined plain [1, 2, 3, 4].

From the micro-geometry point of view, surfaces can be divide into:

• oriented surface – in two perpendicular directions has notably different values of roughness (anisotropic surface) and

• non-oriented surface – the roughness and spacing of peaks in two perpendicular directions are not notably different (isotropic surface).

On the basis of the stated facts, it is possible to classify the blasted surface like non-oriented (isotropic) surface, while its creation is mainly conditioned by abrasive particle shape used, Figure 1. The selection of a type, size, and shape of blasting material depends on a purpose of newly created surface usage. This selection is the basic issue for the process of blasting.

Figure 1.

The classification of surfaces in terms of microgeometry.

Juvenile surface, obtained by a blasting process, has a specific character. The micro-geometry of a blasted surface depends on:

• blasted material properties (mainly hardness). A material with a higher roughness will not be during the blasting from the point of view of its surface micro-geometry affected in such amount, as a soft material;

• a type of used blasting material. Blasting abrasives with a high hardness and with a bigger particle size roughen a surface more intensively compared with fine-grained blasting materials. It is evident that during a blasting process with a demand for obtaining a specified roughness, it is necessary to select an adequate combination of blasted material hardness with hardness and particle size of a blasting material. The ratio of hardness of blasting material to a hardness of blasted metal is important from the point of view of surface qualitative affection as well as in terms of removal of base material;

• blasting parameters. In selection of blasting material type, blasting parameters are also important. The same blasting material creates various surface roughnesses in the different blasting parameters (an abrasive speed, an impact angle, etc.) [5, 6, 7].

Blasting abrasives are loose materials of granular nature consisting of different large particles (polydispersion). There are mainly two types of blasting media used—metallic and nonmetallic ones. Metal blasting abrasives can be in three different forms:

• blasting abrasive of spherical shape, known as shot,

• blasting abrasive of irregular sharp shape – grit,

• blasting abrasive of a roller shape, its height is equal to its diameter – chopped wire.

The shape of particle of blasting abrasives used determines the resulting surface relief [4, 8, 9]. Surface has rounded small peaks after the blasting with grit or sharp notches after grit, Figure 2.

Figure 2.

The surface of metallic substrate blasted by polydispersed (a) shot, and (b) grit.

2. The evaluation of blasted surfaces roughness

Functional properties of areas are dependent on the various surface properties and subsurface layers of a base substrate. One of the criteria concerning the quality of products in terms of production genesis is their geometric and dimensional accuracy, then the surface quality, particularly its roughness. The ISO plan was created for such a purpose creating the structure of necessary norms and standards for the area of “Geometrical specification of products.”

From the point of view of a component function, geometrical properties of a surface are very important in many cases (e.g., in conditions of friction and wear). Therefore, it is necessary to put a sufficient emphasis on the evaluation of surfaces geometry and the determination of profile deviations from the defined plain [10, 11, 12, 13].

The experimental analysis used various blasting materials [5]:

• steel grit (SG) – angular type blasting material is produced by crushing specially treated steel shot. The advantage of this abrasive lies in its durability and impact resistance. Chemical composition: C – 0.75–1.20%, Mn – 0.60–1.10%, Si – 0.60–1.10%, P – max 0.04%, S – max 0.04%.

• steel shot (SS) – is made from hypereutectoid specially processes steel. It has fine homogeneous structure of tempered martensite, which exhibits optimal resilience and resistance to fatigue. Chemical composition: C – 0.75–1.20%, Mn – 0.60–1.10%, Si – 0.60–1.10%, P – max 0.04%, S – max 0.04%.

• brown corundum (BC) – is very aggressive and hardest material used for blasting. It is made by fusing bauxite in the induction furnace at a temperature of 1600°C. It is used for cleaning steel or gray cast iron, the removal of the tip of refined steel, the processing of wood and plastic, rust removal, roughening, tarnishing. Despite its mineral nature it is not silicogenic. Chemical composition: Al₂O₃ – 95.50% min, SiO₂ – 1.40% max, Fe₂O₃ – 0.60% max, CaO – 0.40% max, TiO₂ – 1.80–2.80%.

The type of material whose surface has been subjected to the surface roughness analysis was the steel plate S235JRG2 (EN 10025), thickness 2 mm. It is a non-alloy high-quality structural steel, it can be cold formed and after subsequent normalization even in heat, it is suitable for welding. Chemical composition: C – 0.19% max, Mn – 1.50% max, P – 0.045% max, S – 0.045% max, N – 0.014% max. The dimensions of the testing samples were 100 × 30 mm.

The blasting was pneumatic air-blast device by equipment TJVP 320, producer Škoda, Plzeň, Czech Republic. The samples were blasted with abrasives at working pressure of 0.6 MPa, nozzle diameter was 1.2 mm, blasting angle 75° and the distance of the nozzle from the substrate was 200 mm.

2.1 2D surface roughness parameters and characteristics of blasting surface

The basis of measurement 2D surface roughness parameters and characteristics of surface – the planer evaluation (in direction x – y) is a unit of length (μm). ISO 4287:1997 standard – nowadays the valid international standard defines terms and surface roughness parameters. In this standard, the calculating system for the evaluation of surface roughness parameters is based on the system of the mean line of the roughness profile, the waviness, and the mean line of the primary profile.

2D roughness measurement is often performed using a contact profilometer Surftest SJ – 301, Mitutoyo, Tokyo, Japan. Settings in roughness measurement:

• a measured profile: R,

• a filter: GAUSS,

• a sampling length l (λc): 2.5 mm,

• a number of sampling lengths: N = 5,

• an evaluation length ln: 12.5 mm,

• a number of measured profiles: 20.

The probe stylus of a device is in direct contact with the evaluated surface, Figure 3.

Figure 3.

The contact between a profilometer stylus and measured surface.

The sharp stylus of the profilometer sensor converts the distribution on the surface roughness on mechanical movement, which is then processed by the sensor into an electrical signal and further interpreted as the numerical value of the selected surface roughness parameters of micro-geometry, as well as the graphic record of the surface roughness profile – profilograph.

The some important surface roughness parameters of ISO 4287:1997 are:

• Ra – Arithmetical mean deviation of the assessed profile—indicates the average of the absolute value along the sampling length, Figure 4.

• Rz – Maximum height of the profile—indicates the absolute vertical distance between the Maximum profile peak height and the Maximum profile valley depth along the sampling length, Figure 5.

• RSm – Mean width of the profile elements—indicates the average value of the length of the profile element along the sampling length, Figure 6. Xsi is the length of a single profile element.

Figure 4.

Arithmetical mean deviation of the assessed profile Ra.

Figure 5.

Maximum height of the profile Rz.

Figure 6.

Mean width of the profile elements RSm.

In Table 1, values of averages of selected surface roughness parameters of blasted surfaces using various blasting materials are listed as steel grit (SG) with particle size dz. = 0.71 mm, brown corundum (BC) dz. = 0.9 mm, and steel shot (SS) dz. = 0.9 mm. The arithmetic average was calculated from 20 measured.

Blasting materials	Ra (μm)	Rz (μm)	RSm (μm)	RPc (−/cm)
SS 0.9	11.22 (±0.87)	60.79 (±4.54)	642.70 (±72.27)	15.88 (±2.15)
SG 0.71	10.91 (±0.68)	68.70 (±6.62)	355.05 (±45.12)	28.60 (±4.08)
BC 0.9	11.43 (±0.96)	70.29 (±7.12)	339.45 (±39.25)	29.21 (±3.95)

Table 1.

Arithmetic average of surface roughness parameters of blasted surfaces [5].

For all the evaluated surfaces, a comparable surface roughness parameter Ra was achieved; however, values of other surface roughness parameters (Rz – maximum height of profile, RSm – mean width of the profile elements, RPc –peak count per a length unit) are different, and it depends on used type of a blasting material. Individual profiles with material ratio curves of blasted surfaces also show differences between the surfaces, Figure 7.

Figure 7.

The profiles of surfaces blasted with different blasting materials [5]. (a) the profilograph of a surface blasted by steel shot, (b) the profilograph of a surface blasted by steel grit, and (c) the profilograph of a surface blasted by brown corundum.

Differences in the micro-geometry of surfaces blasted by BM of different shape (rounded and sharp-edged) are concisely expressed by profilographs and material ratio curves (Abbot Firrestone curves), Figure 7. By comparing the material ratio curves of the assessed profile it is possible to explain the difference between surfaces blasted by sharp-edged and rounded blasting material and therefore by the course of material ratio curve in upper level of cut (cca 40%). Bigger material portion in the level is achieved at surfaces which are blasted by sharp-edged devices, what creates more suitable conditions for good anchoring of subsequently applied coatings.

The shape of valleys at surfaces blasted by sharp-edged blasting materials also play a notable role (formation of wedge or anchor profile elements) in anchoring the coatings into more rugged and more heterogeneous reliefs regarding the shape, which are achieved for those surfaces.

In the complex appraisal of the blasted surfaces micro-geometry an evaluation by a set of chosen surface roughness parameters is necessary. According to the analysis, the most suitable surface roughness parameters are:

• Ra – Arithmetical mean deviation of the assessed profile,

• Rz – Maximum height of the profile,

• RSm – Mean width of the profile elements (eventually RPc – the peak count per the unit of length),

• The material ratio curve of the profile (Abbot Firrestone curves).

By the combination of those surface roughness parameters it is possible to expertly distinguish differences in micro-geometry of blasted surfaces, what is important in terms of further technologies, which led to their creation [11, 14].

2.2 3D surface roughness parameters and characteristics of blasting surface

The complex information about a surface is possible to gain by three-dimensional, therefore spatial, measuring of the surface profile. Spatial 3D surface characteristics allow, compared with flat 2D characteristics, to evaluate and define a surface more detail.

Spatial measuring and evaluation of a surface 3D, brings very valuable and practically usable information about relations between geometrical characteristics of a surface and its functional properties. Fast development of measuring technique and control software will nowadays allow implementing advantages of 3D evaluation for a surface texture. Spatial measuring of a surface micro-geometry in comparison with 2D evaluation of a single profile assures more objective presentation of the whole surface with notably bigger statistical meaning of obtained characteristics. Individual spatial surface roughness parameters of a texture are calculated from considerably bigger amount of data.

Data for spatial evaluation of the surface texture are possible to obtain either by a contact measurement (scanning of a set of normally parallel profiles) or by the optical technique. Optical devices use scanning ray, which monitors a surface similarly as a contact scanner or defined viewing field of a microscope.

Nowadays a variety of devices are available (contact and optical) using which it is possible to measure also a surface texture. Differences in measurement are given by various principles of scanning systems, varied precision of measurement, and also interaction of devices and controlled surface. Difference in results is also affected by the methodology of verification of devices properties and their calibration.

ISO 25178 norm is an norm for spatial texture of 3D surface. Measuring and elaboration of notably bigger amount of data, which describe a spatial surface profile, bring a huge amount of information for a real presentation of the controlled surface.

Practical result of implementation of the norm is not only a contribution for the selection of suitable surface roughness parameters of surface structure evaluation, but also a preparation of measuring and analyzing devices of a quantitative control of surface structure. Advantages of spatial evaluation show that they are progressive metrological methods. Not only increasing requirements on precision and quality of existing production technologies, but also the development of new materials and technological methods will undoubtedly contribute for their wider practical utilization.

A 3D study of blasted surfaces of different types of blasting abrasives (shot and grit types) in a contact way is shown in Figure 8.

Figure 8.

3D imaging blasted surfaces by contact method. (a) 3D surface blasted by steel shot, (b) 3D surface blasted by steel grit, and (c) 3D surface blasted by brown corundum.

The creation of a 3D surface texture by the contact method was realized using the Surftest SJ – 301 stylus profilometer, Mitutoyo, Japan. The measured research area was 4 × 4 mm. The resulting spatial image of the surface is then created in such a way that the profile curves and their data in the parallel direction at a defined scanning distance are recorded by the scanning tip of the profilometer and subsequently joined by the software. The method is applicable if the workplace has only a 2D roughness meter. The disadvantage of the method is its time-consuming compared to contactless evaluation.

3D imaging of blasted surfaces was performed by the non-contact method with a confocal laser microscope Olympus LEXT OLS 3000, Figure 9. The measured research area was 1.25 × 1 mm, the profile height is presented on the Z axis.

Figure 9.

3D imaging of blasted surfaces by the non-contact method. (a) 3D surface blasted by steel shot, (b) 3D surface blasted by steel grit, and (c) 3D surface blasted by brown corundum.

It is a highly accurate 3D measurement in real time and a reliable evaluation of profiles, which is based on the illumination of the sample with a laser beam, which is focused on one point. This eliminates noise caused by unwanted light, resulting in better image quality.

The performed measurements show that noncontact method is sensitive enough for imaging blasted surfaces. Compared with the contact method, the contactless method is less time-consuming, but the device itself is more expensive compared with the conventional roughness tester.

At a mutual visual comparing of assessed surfaces, Figures 8 and 9, a difference is visible in a character of surfaces blasted by various blasting materials. At such types an uneven surface is reached, resulting by an incidental fall of particles—blasting abrasives. A surface blasted by a sharp blasting abrasives—steel and corundum grit does not show such uniformity as at blasting by round particles—steel shot. Notches are on the surface in a various orientation, intersected mutually and a notable part of holes and juts is sharp. From utilized sharp blasting materials, the most segmented surface was detected at the surface blasted by a steel grit. At blasting by round blasting abrasives—a steel shot, more uniformed surface modification was achieved, which is created by intersected round juts.

By realized measurements and 3D visualization of blasted surfaces a presumption was proved that abrasive particles make after the fall in a base material their prints which are dependent on their shape and size, by which a detection of surface complexity was achieved—thus its various segmentation. The particle shape of blasting abrasives (round or angular) has thus a notable effect in the process of blasting and is one of the attributes for an achieved microgeometry of blasted surface and further characteristics as well.

3. Influence of blasting abrasive and technological parameters on the roughness of blasted surface

The main factors that may affect the quality and thus the roughness of the blasted surface are as follows:

• the abrasive shape and particle size,

• the abrasive hardness,

• the abrasive particle size distribution,

• the blasting process parameters—abrasive velocity, blasting distance, impact angle, hardness of substrate [5].

3.1 Impact of abrasive shape

The nature of blasted surface depends on the shape of abrasive used. When using round particle of blasting abrasive (shot), relatively uniform deformation of the surface is achieved. The surface consists of intersecting spherical dimples, Figure 18a. Sharp angular particles of abrasive (grit) cause notches in the substrate whose orientation on the surface is stochastic. Particles in their random movement in the stream of abrasive can impact the surface by their edge, straight surface or tip, Figure 10.

Figure 10.

Surface of mild steel after blasting after the impact of (a) round, and (b) sharp angular blasting abrasive.

3.2 Impact of abrasive particle size

Large particles concentrate significant impact energy at the point of impact while their effect depends on the use of this impact energy. Impacting particle causes not only local disintegration of oxide layer but also significantly hammers the substrate. The effect on the substrate is reflected mainly in its plastic deformation and corresponding work-hardening, or depending on the impact angle also the removal of the substrate occurs. With larger particles of abrasive a higher surface roughness is achieved, but also a greater amount of blasting abrasive is required for complete surface coverage.

Small particles, on the other hand, have smaller kinetic energy which is consumed mainly for removal of scale layer from the surface and it has less influence on the substrate. Smaller particles require lower necessary quantities and the surface is smoother and evenly covered. Surfaces blasted by different particle sizes of blasting abrasives are shown in Figure 11.

Figure 11.

Surfaces of mild steel blasted by different particle sizes of blasting abrasives (SS – Steel shot, SG – Steel grit) [6].

3.3 Impact of abrasive hardness

Impact of abrasive hardness on the substrate is reflected in character of blasted surface. Impact of substrate hardness is suppressed when blasting by very hard abrasives. When selecting blasting abrasive for pretreatment of the substrate, the following principle applies: selected abrasive must be softer than the material to be retained after blasting but it has to be harder than the material that is meant to be removed by blasting.

3.4 Impact of abrasive particle size distribution

The homogeneity of particle size distribution affects the appearance and roughness of the blasted surface. The narrower is the particle size distribution, the more marked is the surface relief. In wide particle size distribution the characteristic surface relief is not so marked.

The appearance of surfaces blasted by poly-dispersed mixtures of blasting abrasives (steel shot and chilled iron grit) is shown in Figure 12 [5].

Figure 12.

The appearance of mild steel surfaces blasted by polydispersed mixtures of (a) steel shot, and (b) iron grit [5].

3.5 Impact of abrasive velocity

From Figure 13, it is clear that the higher the velocity of the particle, the higher the roughness of the substrate and the less the blasting abrasive necessary to cover a certain substrate area. To achieve high quality of surfaces one chooses greater blasting velocity for a larger abrasive particle size and less blasting speed for smaller particle size [5].

Figure 13.

Dependence of Ra on particle velocity v at various particle sizes [5].

3.6 Impact of blasting distance

The effect of nozzle-substrate distance on surface roughness is not clearly determined. While some tend to believe that the distance of the nozzle from the surface does not affect the resulting surface roughness, others have found a clear, almost a linear, increase in roughness with an increasing nozzle-substrate distance. It has been found that nozzle-substrate distance affects the final roughness only when a specific distance is exceeded. The value of the specific distance depends on air pressure, for instance for a pressure of 0.4 MPa, the critical distance is 250 mm. It is possible to determine an optimal nozzle – substrate distance for achieving the maximal Ra. This optimal distance depends on the hardness of the substrate, Figure 14. As abrasive blasting material used corundum with grain size 1.4 mm. It is very aggressive and hardest material used for blasting (more than 9 Mohs scale).

Figure 14.

Influence of nozzle-substrate distance on surface roughness for substrates with low and high hardness; optimal range of blasting distance indicated by dotted line [5].

For materials with higher hardness the optimal range of nozzle-substrate distance is wider. At a distance greater than the optimal, less kinetic energy of the impacting particles causes a decrease in roughness.

3.7 Impact of hardness of substrate

Figure 15 shows that with increasing hardness of substrate at constant blasting conditions a decrease in blasted surface roughness occurs [6].

Figure 15.

Dependence of blasted surface roughness Ra on the substrate Vickers hardness (HV) for different abrasive particle sizes [6] (1 – Low particle size, 6 – High particle size).

Depending on the combination of substrate and abrasive hardness, four different states can occur:

1. abrasive and substrate are relatively soft – deformation of both occurs,

2. abrasive is relatively soft, substrate is relatively hard – abrasive deformation is significant, substrate will be polished,

3. abrasive is relatively hard, substrate is relatively soft – substrate is fully covered by notches or dimples,

4. both abrasive and substrate are relatively hard – intense fragmentation of the blasting particle material occurs with only little roughening of substrate.

The resulting surface morphology is determined by the type of substrate, material of blasting abrasive and blasting conditions.

3.8 Impact of blasting angle

Blasting angle α for air blasting is characterized as the angle between the surface of the substrate and blasting stream (Figure 16a) or for a mechanical blasting it is the angle between the trajectory of particles flying out of blasting wheel and the substrate (Figure 16b).

Figure 16.

Depiction of blasting angle for (a) air blasting, and (b) mechanical blasting (BW – Blasting wheel, v – Abrasive speed, 1–8 blasted samples positioned around blasting wheel).

Blasting angle affects changes caused by the impact of blasting abrasive. During blasting, either mechanism of creating indentations or removal or grooving mechanism prevails. If the blasting angle is less than 45°, the grooving effect of abrasive prevails and the length of the grooves is higher, the smaller the blasting angle is. The surface of the substrate with grooves does not lead to good strength of bonded joints. At a blasting angle of 75° removal mechanisms prevail and the resulting surface is roughened containing many sites suitable for mechanical anchoring of the adhesive. At a blasting angle of 90° indentation mechanism prevails and the resulting surface is not suitable for mechanical anchoring of the adhesive.

Values of roughness along the blasted area are not constant. In the middle of the track, roughness reaches the highest value due to the fact that the stream of accelerated particles is stable there. At the periphery of the track, the particles are deflected from their trajectory by other particles bounced back from substrate, and moreover the roughness values are smaller Figure 17.

Figure 17.

Distribution of surface roughness in affected area (steel grit, blasting angle 30°, particle size 0.9 mm).

The change in surface roughness at different impact angles is shown in Figure 18. At an impact angle of 75° Ra value of the surface is maxim. The impact angle of 75° is an optimum angle at which the maximum material is removed, and cleaning and roughening occurs with minimum consumption of abrasive [5].

Figure 18.

Dependence of Ra on impact angle of steel grit with different particle sizes [5].

4. Conclusions

The production history of materials leaves characteristic features on the surface that affect their behavior during other technological processes. This chapter is focused on the issue of morphological changes that occur on surfaces after abrasive blasting. These changes in terms of roughness need to be monitored as accurately as possible with the whole set of surface roughness parameters, not only from a 2D but also a 3D perspective. This gives a comprehensive view of the blasted surface. At the same time, the factors that contribute to these changes must be taken into account. These are mainly the technological conditions of blasting and the type of blasting abrasive. By their suitable choice, it is possible to achieve the most suitable surface pretreatment by abrasive blasting with the required morphological, tribological, and other properties.