Find all positive integers $ k$ such that for any positive numbers $ a, b$ and $ c$ satisfying the inequality 

\[ k(ab \plus{} bc \plus{} ca) > 5(a^2 \plus{} b^2 \plus{} c^2),\]

there must exist a triangle with $ a, b$ and $ c$ as the length of its three sides respectively.
To solve the problem, we need to find all positive integers \( k \) such that for any positive numbers \( a, b, \) and \( c \) satisfying the inequality 

\[ k(ab + bc + ca) > 5(a^2 + b^2 + c^2), \]

there must exist a triangle with \( a, b, \) and \( c \) as the lengths of its three sides respectively.

1. **Triangle Inequality Condition**:
   The condition that \( a, b, \) and \( c \) are sides of a triangle is equivalent to the condition that \( a, b, \) and \( c \) satisfy the triangle inequalities:
   \[
   a + b > c, \quad b + c > a, \quad \text{and} \quad c + a > b.
   \]
   Alternatively, we can express \( a, b, \) and \( c \) as:
   \[
   a = x + y, \quad b = y + z, \quad \text{and} \quad c = z + x,
   \]
   for some positive real numbers \( x, y, \) and \( z \).

2. **Substitute and Simplify**:
   Substitute \( a = x + y \), \( b = y + z \), and \( c = z + x \) into the given inequality:
   \[
   k((x+y)(y+z) + (y+z)(z+x) + (z+x)(x+y)) > 5((x+y)^2 + (y+z)^2 + (z+x)^2).
   \]
   Expanding both sides, we get:
   \[
   k(xy + yz + zx + y^2 + yz + zx + z^2 + zx + x^2 + xy + zx + x^2 + xy + y^2) > 5(x^2 + 2xy + y^2 + y^2 + 2yz + z^2 + z^2 + 2zx + x^2).
   \]
   Simplifying further:
   \[
   k(2x^2 + 2y^2 + 2z^2 + 3xy + 3yz + 3zx) > 5(2x^2 + 2y^2 + 2z^2 + 2xy + 2yz + 2zx).
   \]
   This reduces to:
   \[
   k(2(x^2 + y^2 + z^2) + 3(xy + yz + zx)) > 10(x^2 + y^2 + z^2 + xy + yz + zx).
   \]

3. **Rearrange and Compare Coefficients**:
   Rearrange the inequality:
   \[
   2k(x^2 + y^2 + z^2) + 3k(xy + yz + zx) > 10(x^2 + y^2 + z^2) + 10(xy + yz + zx).
   \]
   Grouping like terms:
   \[
   (2k - 10)(x^2 + y^2 + z^2) + (3k - 10)(xy + yz + zx) > 0.
   \]

4. **Analyze the Coefficients**:
   For the inequality to hold for all positive \( x, y, \) and \( z \), the coefficients of \( x^2 + y^2 + z^2 \) and \( xy + yz + zx \) must be non-negative:
   \[
   2k - 10 \geq 0 \quad \text{and} \quad 3k - 10 \geq 0.
   \]
   Solving these inequalities:
   \[
   2k - 10 \geq 0 \implies k \geq 5,
   \]
   \[
   3k - 10 \geq 0 \implies k \geq \frac{10}{3}.
   \]
   Since \( k \) must be an integer, the stricter condition is \( k \geq 10 \).

Therefore, for all positive integers \( k \geq 10 \), the inequality holds, ensuring that \( a, b, \) and \( c \) can form the sides of a triangle.

The final answer is \( \boxed{ k \geq 10 } \).