Abrasive jet micro-machining (AJM), in which abrasive particles are accelerated by air and directed toward a target, has been used to make components for micro-electro-mechanical (MEMS) and micro-fluidic capillary electrophoresis devices. One of the disadvantages of AJM is that the compressed air jet used to propel the erodent particles diverges significantly after the nozzle exit, increasing the size of the blast zone and the width of the smallest channel or hole that can be machined without the use of a patterned erosion-resistant mask that defines the micro-feature edges. Abrasive slurry jet micro-machining (ASJM) is similar to AJM except that pressurized water, instead of air, is used to accelerate the suspended abrasive particles such as garnet or alumina (Al2O3). In both AJM and ASJM, the material removal occurs by erosion. However, for the same jet dimension and low speed, slurry jets have a much lower divergence angle than air jets, allowing for the micro-machining of small features without the use of patterned masks.

1. DEPT. OF MECHANICAL ENGINEERING 1
CHAPTER 1
INTRODUCTION
Abrasive jet micro-machining (AJM), in which abrasive particles are accelerated by
air and directed toward a target, has been used to make components for micro-electro-
mechanical (MEMS) and micro-fluidic capillary electrophoresis devices. One of the
disadvantages of AJM is that the compressed air jet used to propel the erodent particles
diverges significantly after the nozzle exit, increasing the size of the blast zone and the width
of the smallest channel or hole that can be machined without the use of a patterned erosion-
resistant mask that defines the micro-feature edges. Abrasive slurry jet micro-machining
(ASJM) is similar to AJM except that pressurized water, instead of air, is used to accelerate
the suspended abrasive particles such as garnet or alumina (Al2O3). In both AJM and ASJM,
the material removal occurs by erosion. However, for the same jet dimension and low speed,
slurry jets have a much lower divergence angle than air jets, allowing for the micro-
machining of small features without the use of patterned masks.
High pressure ASJM was investigated by Miller, who demonstrated its use for the
cutting of micro-slots at 70 MPa through the thickness of thin sheets of metals, glass,
ceramics, and polymer sand composite materials. Further studies demonstrated that relatively
low pressure ASJM (214 MPa) is a practical process not only for cutting, but also for the
milling or etching of materials. Using dimensional analysis and multi-variable regression of
experimental data, Pang et al. modelled the erosion rate, opening width and wall slope of
machined channels in glass. They found that the channels suffered from waviness due to the
mechanical vibration of the equipment. The roughness of micro-channels can affect fluid low
phenomena such as electro-osmotic mobility, separation efficiency and solute dispersion in
micro-fluidic applications, which makes it essential to develop models in order to predict the
roughness of micro-channels for a variety of machining conditions.

2. DEPT. OF MECHANICAL ENGINEERING 2
CHAPTER 2
LITERATURE SURVEY
Miller [2004], who demonstrated high pressure ASJM for the cutting of micro-slots
at 70 MPa through the thickness of thin sheets of metals, ceramics composite materials.
Jafar [2013], developed a numerical model to simulate the brittle erosion process in
AJM and predict the steady-state roughness and erosion rate of channels in borosilicate glass.

3. DEPT. OF MECHANICAL ENGINEERING 3
CHAPTER 3
OBJECTIVES
The aim of the present paper was to develop a model to predict the roughness and
erosion rate of unmasked channels machined with abrasive liquid slurry jets in borosilicate
glass. The models that were previously developed for abrasive air-jet machining formed the
starting point for the work.

4. DEPT. OF MECHANICAL ENGINEERING 4
CHAPTER 4
METHODOLOGY
4.1 EXPERIMENTAL SETUP
All the experiments were conducted using the low-pressure ASJM system, which is
capable of micro-machining channels and holes with a high degree of repeat-ability and a low
waviness. As shown in Fig. 1, the main components of the system are an open-reservoir
slurry mixing tank, a positive displacement slurry pump with pulsation damper, a 180
sapphire orifice, and a computer controlled linear stage to control movement of the target
relative to the stationary nozzle. The linear stage was capable of speeds up to 7 mm/s with a
step size of 0.047 producing a smooth, repeatable motion. For all experiments, the orifice to
target standoff distance, do, along the orifice centre-line was maintained constant at 20 mm,
and 25 percentage nominal diameter alumina (Al2O3) particles were mixed with water at
various concentrations to form the slurry.
Fig. 4.1: Schematic of the abrasive slurry jet apparatus
All micro-machining was conducted on 100 mm 50 mm 3 mm thick borosilicate glass
plates having a Young’s modulus (E) of 63 GPa, Poisson’s ratio of 0.2, fracture toughness
(Kc) of 0.76 MPa, and a Vickers hardness (H) of 5.4 GPa. Straight, unmasked channels were
machined at various nozzle pressures, P, using two targets traverse speeds, versus and six jet

5. DEPT. OF MECHANICAL ENGINEERING 5
impact angles, (Fig. 2) at each pressure, as shown in Table 4.1. The orifice discharge
coefficient (ratio of the actual mass low rate at the orifice exit to that of an ideal orifice) was
measured to be 0.60 0.03 which reduced the actual jet diameter to 140.Table 4.2 gives the
measured jet low rates and the calculated velocities used in the channel machining and single
impact experiments.
Fig. 4.2: Schematic of jet orientation during erosion rate measurements
Table 4.1: Process parameters used in the machining of the channels and the single impact
experiment.
PARAMETERS CHANNEL MACHINING SINGLE IMPACT
EXPERIMENT
Pressure, P (MPa) 6.1,4.7,3.6,3.0,2.6,2.1 4.7
Traverse speed, vs (mm/s) 0.2,0.3 3.5
Mixing tank particle
concentration, C (wt%)
0.25 0.01
Jet Impact angle, θ (deg) 15,30,45,60,75,90 90
Table 4.2: Slurry flow rate and jet velocities based on the applied pressure.
PARAMETERS CHANNEL MACHINING SINGLE IMPACT
EXPERIMENT
Pressure, P (MPa) 6.1,4.7,3.6,3.0,2.6,2.1 4.7
Slurry flow rate, Q (mL/s) 1.7,1.5,1.3,1.2,1.1,1.0 1.5
Jet velocity, vf (m/s) 110,96,89,78,72,65 97

6. DEPT. OF MECHANICAL ENGINEERING 6
CHAPTER 5
RESULTS AND DISCUSSIONS
5.1 SINGLE IMPACT EXPERIMENTS AT 90 DEGREE
Individual craters on borosilicate glass after blasting 25 mu particles at P= 4.7 MPa
and 90 Degree incidence using a scanning speed of 3.5 mm/s and a low slurry particle
concentration of 0.01 wt Percentage is shown in the figure given below.
Fig. 5.1: Individual craters on borosilicate glass
An area of 0.5 mm X 2 mm of the surface subjected to sparse single impacts at a
pressure of 4.7 MPa (Table 1) was scanned using the optical profilometer, and 986 craters
were counted and measured within the scanned area, implying that about 40 Percentage of the
incoming particles damaged the target. Fig. 9 illustrates a typical 3D scan of the surface after
the single particle impact tests. The shapes of the craters were similar to those found using air
jets, suggesting that brittle erosion was also the dominant damage mechanism in the slurry
jet. Based on the theoretical threshold (38.8 nJ), 28 Percentage of the particles in the
distribution should have initiated lateral cracks in target, while if the lower apparent threshold

7. DEPT. OF MECHANICAL ENGINEERING 7
based on Wensinks work was used (19.4 nJ), the percentage increased to 58Percentage.
Therefore, these two estimates of the fraction of incident particles with sufficient energy to
generate craters bound the measured value of 40 percentages.
5.2 OBLIQUE BLASTING
Measured channel centreline roughness and erosion rate of borosilicate glass as a
function of impact angle at P = 4.7 MPa as shown in the below figure. The error bars show
the absolute range of the data over three measurements.
Fig.5.2: Measured channel centreline roughness and erosion rate of borosilicate glass
The above Fig. 5.2 shows the measured erosion rates and average roughness for P =
4.7 MPa and the experimental conditions given in Table 1. It is seen that the erosion rate
increased with the nominal impact angle (angle of the free jet relative to the target),
exhibiting a maximum at 90, which is characteristic of brittle erosion Similar trends were
found for the other pressures and conditions in Table 1. Increases in the impact angle resulted
in higher normal impact forces, leading to larger chips being removed, and ultimately higher
erosion rates and rougher channels.

8. DEPT. OF MECHANICAL ENGINEERING 8
5.3 COMPARISON OF THE ANALYTICAL MODELS
The comparison of the analytical models with the experiments is as
follows:
5.3.1 COMPARISON OF THE ANALYTICAL MODELS WITH SURFACE
ROUGHNESS AT NORMAL INCIDENCE
The below Fig. 5.3 shows the measured roughness of the ASJM channels as a
function of the incident effective average particle kinetic energy at normal incidence rougher
channels were produced when particles of higher average normal kinetic energy impacted the
surface, since they produced larger chips. The middle solid line in Fig. 5.3 shows the least-
squares power law _t of the experimental data for all the conditions of Table 1 (Ra =
0.43U0.20 with a coefficient of determination (Square of R) of 0.97) and the dashed line is
the equivalent trend line through the AJM experimental data of (Ra = 0.42U0.20 where U is
in nJ). U for the AJM data trend line was found using the average particle size and average
velocity of all particles in the jet, since the kinetic energy of all the particles was above the
apparent threshold. The difference between these two trend lines is only 2 Percentage,
confirming that, despite the differences in particle trajectories and ow patterns between
ASJM and AJM, the resulting roughness was governed only by the normal kinetic energy of
the incident particles above the threshold for lateral cracking, as was found for AJM. Fig. 5.3
also shows that the analytical model I overestimated the roughness with an average
error of 122 Percentage, but that model II, which assumed crack initiation at a depth.
a, resulted in a better agreement with the measured Ra, underestimating with an average error
of 42 Percentage over the entire range of studied kinetic energies. As discussed in the context
of AJM, this underestimation was likely due to the assumption of the crater depth a used in
the model. Although the measured crater depth was indeed closer to location a than b, it was
nevertheless about 22 Percentage larger than the measured indentation depth. Increasing a by
22 Percentage in model II increased the roughness (not shown in Fig.5.3) and lowered the
average error in the predicted roughness to 17 Percentage.

9. DEPT. OF MECHANICAL ENGINEERING 9
Fig. 5.3: Measured steady-state roughness of borosilicate glass channels machined using
ASJM compared to predictions of the analytical models
5.3.2 EROSION RATE AT NORMAL INCIDENCE
The below figure 5.4 shows that, as expected, the erosion rate increased with
increasing Ueff. As with the roughness predictions, model II agreed better with the data than
model I, having an average error of 41 Percentage compared with 232 Percentage for model
I. Just as with the underestimate of roughness, model II underestimated the erosion rate,
probably because the predicted crater depth was too small. For example, increasing the crater
depth a by 22 Percentage in model II to match the experimental measurements, reduced the
average error in the predicted erosion rate to 12 Percentage.

10. DEPT. OF MECHANICAL ENGINEERING 10
Fig. 5.4: Erosion rate of borosilicate glass as a function of average effective particle and
normal kinetic energy
5.3.3 OBLIQUE BLASTING RESULTS
The experimental data are reproduced from Fig. 5.1 and the predictions at 90 degree
are reproduced from Figs. 5.3 and 5.4. The predictions at 30 Degree and 60 degree were very
similar to those at 90 Degree (Figs. 5.3 and 5.4) in that model I overestimated the roughness
and erosion rate while model II underestimated those parameters although with a lower
average error. The errors in roughness prediction for 30 Degree and 60 Degree were 49
Percentage and 41 Percentage in Model I and 101 Percentage and 126 Percentage in Model
II, respectively. Similarly, the errors pertaining to the predictions of erosion rate for 30
Degree and 60 Degree were 48 Percentage and 27 Percentage in Model I and 292 Percentage
and 231 Percentage in Model II, respectively.

11. DEPT. OF MECHANICAL ENGINEERING 11
Fig. 5.5: Measured and predicted centreline channel roughness and (b) erosion rate of
borosilicate glass
5.3.4 CFD MODEL RESULTS
The CFD models were validated by comparing with published experimental data for
the velocity of free water jets and the pressure distribution on a target surface under an
impacting jet. In all cases, the agreement was very good. The CFD model was executed for
30 Degree and 90 Degree global impact angles, and in each case, separate simulations were
conducted over the range of applied pressures (e.g. 2.1, 2.6, 3.1, 3.6, 4.7 and 6.1 MPa). Fig.
5.6 and 5.7 illustrate the water velocity and particle trajectories distributions close to the
target in the 90 Degree orientation for 25 mu meter particle at a jet velocity of 89 m/s (i.e. P =
3.6 MPa, Table 4.2). Particle and fluid velocities were equal from the inlet of the domain to
within 1 mm of the target wall, where the stagnation zone quickly decreased the particle
velocity. The 25 mu meter particles, regardless of their position in the jet, impacted the target,
rebounded, were entrained by the lateral low parallel to the target and carried away from the
jet footprint. The variation of the drag force within the stagnation zone caused the particle
impact velocity and angle to be 49 m/s and 90 Degree at the centre of the jet, and 55 m/s and
43 Degree at the edge of the jet, as shown in Fig. 5.6. When applied to the present situation,
the analysis of Humphrey predicts such behaviour for most particles that are less than
approximately 10 mu meter in diameter. An increase in the viscosity of the slurry would

12. DEPT. OF MECHANICAL ENGINEERING 12
increase the momentum equilibration number, as defined by Humphrey, causing a greater
tendency for particles to follow the streamlines. This in turn would decrease the local impact
angle at the bottom of the channel and reduce the amount of erosion. The erosion rate would
also be decreased by the greater drag on particles in the stagnation zone. It was observed that
10 Percentage increase in the slurry viscosity (by adding glycerine to water) resulted in 19
Percentage reductions in channel depth.
Fig.5.6: Slurry jet velocity distribution at 90 Degree
Fig. 5.7: Slurry jet velocity distribution at 30 Degree
The above Fig. 5.7 shows that in the case of oblique jet blasting (less than 90 Degree), the
stagnation zone (i.e. the region where the velocity of the water was less than 10 Percentage of
the incident jet velocity) shifted away from the centreline of the jet. This contributed to an

13. DEPT. OF MECHANICAL ENGINEERING 13
asymmetric variation of the drag force that decelerated the incoming particles prior to impact.
Accordingly, the particle impact velocities varied significantly along the jet footprint. Table
4.3 gives the normal impact velocity ranges and their averages for the different particle sizes
at global impact angles of 30 Degree and 60 Degree.
Table 5.1: CFD predictions of the normal component of impact velocity for particles
PARTICLE
SIZE (µM)
IMPACT
ANGLE
(DEG)
NORMAL
IMPACT
VELOCITY
RANGE (M/S)
AVERAGE
NORMAL
IMPACT
VELOCITY
(M/S)
AVERAGE
NORMAL
KINETIC
ENERGY (NJ)
25 30 26-42 37 22
25 60 17-49 39 24
30 30 27-43 39 42
30 60 22-49 41 46
35 30 27-44 40 70
35 60 27-50 43 81
40 30 29-45 41 110
40 60 28-51 45 130
45 30 30-47 42 160
45 60 30-53 47 200
The above table shows the CFD predictions of the normal component of impact
velocity of the particles.

14. DEPT. OF MECHANICAL ENGINEERING 14
CHAPTER 6
CONCLUSION
The roughness and erosion rate of ASJM channels in borosilicate glass were measured
and predicted using existing analytical and numerical models developed for the AJM of
brittle materials. A brittle erosion mechanism was confirmed using an analysis of single
impact sites obtained with a dilute slurry at a high scan speed, and from the measurements of
erosion rate versus angle of the incident slurry jet. The presence of water was found to have a
negligible effect on the formation of chips in borosilicate glass. As with AJM, the particle
kinetic energy due to the normal component of the velocity was the only parameter which
governed the roughness and erosion rate. Slurry jets of higher normal kinetic energy
produced deeper and rougher channels, since they generated greater damage on the target and
removed larger chips from the surface. Decreasing the impact angle (reducing the normal
impact force on the target) resulted in a smoother channel and lower erosion rates, as
expected. As compared to AJM, ASJM Micro machining is faster and simple. Therefore
ASJM is used in machining of metals, glass, ceramics, laboratory instruments etc

15. DEPT. OF MECHANICAL ENGINEERING 15
REFERENCES
[1] Jafar RHM, Papini M, Spelt JK. ‘Simulation of erosive smoothing in abrasive jet
micro-machining of glass’- J Mater Process Technol (2013).
[2] N, Getu H, Sadeghian A, Papini M. ‘Computer simulation of developing abrasive jet
machined pro_les including particle interference’- J Mater Process Technol (2009).
[3] Miller DS. ‘Micro-machining with abrasive water jets’ -J MaterProcess Technol
(2004).
[4] E, Thurre S, Walchiers E, Sayah A, Gijs MAM. ‘The introduction of powder blasting for
sensor and micro system applications’ -Sens Actuators (2000)
[5] Humphrey J. ‘Fundamentals of uid motion in erosion by solid particle impact’- Int J Heat
Fluid Flow (1990)