**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:


## **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**.

#### **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].

## **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

**Figure 11.** *Surfaces of mild steel blasted by different particle sizes of blasting abrasives (SS – Steel shot, SG – Steel grit) [6].*

**Figure 12.** *The appearance of mild steel surfaces blasted by polydispersed mixtures of (a) steel shot, and (b) iron grit [5].*

*Surface Characterization after Blasting DOI: http://dx.doi.org/10.5772/intechopen.103160*

**Figure 13.** *Dependence of Ra on particle velocity v at various particle sizes [5].*

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].

#### **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).

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].

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


**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].*

#### **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).*

d.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**).

*Surface Characterization after Blasting DOI: http://dx.doi.org/10.5772/intechopen.103160*

**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**.

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

**Figure 18.** *Dependence of Ra on impact angle of steel grit with different particle sizes [5].*
